Yes. Yes, I did.
In 3.5, the cost of a typical silver dagger is 22gp which isn’t too expensive. In 5e, they seem to have increased the cost to 100gp though that’s technically to silver an existing weapon so I could see it being a little cheaper if the smith is making it from the start. Note that a “silver dagger” is likely a dagger plated in silver and not 100% silver. In 2nd edition, you could have a weapon silvered for 10x its base price so, again, around 20gp. Point being that, while a silver dagger may be uncommon, it’s nothing breathtakingly rare either. I suppose there’s a question about why it’s in a (black)smith’s shop and not a (silver)smith’s but I’ll leave that on the DM since I suspect it’s just an oversight.
As for why it’s there just lying around, I suspect it’s there for the same reason other houses and shops have stuff lying around – they got snatched away or otherwise disappeared, hence the mystery.
With all respect to Chekov, RPGs aren’t really “stories” in the traditional sense and there’s more world building (often on the fly) which leads to more mundane explanations. The brass ring on the bandit leader’s hand is just a 5gp brass ring and not a Ring of Flight. The finely crafted sword with silver-wired hilt is just a fancy looking sword added to the mercenary’s description for flavor and not a Longsword +2. The crystal stoppered bottle with engraved fish you discover in the wizard’s chest is just a fancy art piece and not a Decanter of Endless Water. Likewise, not everything with a description or object of possible note is a plot hook. Players ask what’s in a room and the DM perhaps provides extra detail to create the illusion of a living world where people own things just because.
That said, of course things often ARE placed for a reason in an RPG and, unless your DM is exceptionally wordy about every place you visit, stuff he brings up is usually worth investigating or thinking its relevant. But that’s not an in-game excuse for a PC to pocket it.
I guess I disagree. I think PC’s know that they are living in the sort of world where stuff is there for a purpose.
Yes, but they should also know that as Jophiel says, often the purpose of a sword with a fancy hilt is just to sell it.
Or, if the players are used to everything being there for a capital-R Reason, to drive them nuts 
It’s there for a purpose, just not necessarily a purpose involving them
I’d consider that metagaming. A certain level of metagaming is unavoidable but, as a DM, I certainly wouldn’t let metagaming be an excuse for player actions. I can’t stop you from avoiding melee combat with an ochre jelly even if that PC has never seen one before but I sure ain’t gonna accept “Well, it was there so it must be important” as a reason for the paladin casually pocketing something.
While a 22 GP (or 100) item isn’t something that would be out of place in a smithy (especially one frequented by adventurers), it is a bit out of place just sitting on a table. I’d expect something that valuable (and easily-carried) to be in a locked display case. For perspective, a D&D gold piece is worth somewhere in the neighborhood of $200 (plus or minus maybe a factor of two, depending on how you’re doing the comparison).
Yes, this does mean that typical adventurers, who casually drop thousands of gold for a single item, are filthy stinking rich.
I took “in a smithy” to mean “in the work area” where non-smithy people are unlikely to be on a usual day. Depending on how everyone disappeared, it could be out because the smith had just finished it and was preparing to have it delivered/picked up or because he was going to have the hilt wrapped or some other work-related reason.
Your gold conversion sounds ridiculously high. $200 per gold would mean that a bog-standard dagger costs the equivalent of $400. And a hand axe, a glorified hatchet, runs a thousand dollars. Hell, a deck of playing cards (5sp) is a hundred bucks and arrows are $10 a shot.
On the other hand, a GP is said to be 1/50 of a pound, which at current rates for gold would be $355. A mug of ale is 4 cp, and a mule (8 GP) can be a few thousand dollars in our world.
When doing this comparison, you want to try to find the most mundane items, because anything mostly bought by filthy rich adventurers is likely to be highly inflated. That’s also why, for instance, a 10 foot pole is more expensive than a 10 foot ladder: Peasants buy ladders, but adventurers buy poles.
In 5e, a 10’ pole is five copper and a 10’ ladder is five silver. And daggers and arrows aren’t really “adventuring gear” in the same sense as a flail or a longsword is. I’d say that a deck of cards counts as pretty mundane. A shovel costs 2gp which means your average peasant farmer is outlaying $400 just to dig holes.
Really, D&D prices don’t really relate to real world prices because they’re meant more for player balance than accurately modeling an economy.
Casually pocketing a dagger while he browses a shop? Of course not. Taking a dagger that is notably lying out in a mysteriously abandoned village? That’s really quite different. He’s on a quest to rescue the missing inhabitants. Weird valuable objects he encounters in the direct exploration of that goal are likely to be relevant to it.
Of course, when he finds the smith, or the heir of the smith, he needs to make a good faith effort to return it. And perhaps he should leave a note when he takes it. But silver daggers aren’t just casually lying around. It’s meaningful.
D&D Economics have never made sense.
I use a figure of $50 per gold piece and then tweak prices based on that. Of course, when I run a game, you can’t just walk into some 5,000 pop town, unload a pile of gold and magic items and then walk away with anything you want out of the books, like most D&D campaigns let you do (for convenience).
Of course, in the Pathfinder GM Guide, a metropolis (the largest city) has a max 8th level spellcasting for hire and that 5k pop town a max 6th level. But try to make it some kind of quest to find a 9th level caster to make some item (and get them to spend 6 months making it by the rules) and watch your players howl at how unfair it is! No, they want to be able to pool their money and buy a Vorpal Scythe +5 right now regardless of where they are!
I said the same thing about the paladin; I wouldn’t ding him for saying “I think this dagger could uniquely aid me in rescuing the town and will return it to the smithy with my thanks once the town is saved”.
But I wouldn’t accept “It’s a dagger and it’s there so you must want me to take it” which is the Chekov’s excuse.
Paper products in general are more expensive in a D&D (or real-world medieval) world than in ours-- That’s probably the reason for the cards. I’d never noticed the price of a shovel, though: That one does look weird, and not really consistent with any of the other items on the lists.
Prices of alchemical and minor magic items are really designed to make players spend money and limit how many they use.
For example, at my $50/gp rate, a 100gp first level potion is $5,000. Honestly, there’d be little incentive to make them other than as emergency use and certainly not for regular healing.
At 18,000 gold for 50 +2 arrows (Or +1 Flaming), that’s $18,000 per arrow. Absolutely absurd that anyone would make those and hand them out like candy.
But yet in the 4e game I played, the archer player bought tons of all sorts of trick arrows, because “I don’t really have anything else to spend money on”.
I (at least) am not saying that the dagger simply being there counts as a reason for the LG character. I’m more asking why the DM felt the need to make the player justify their characters action in the first place. The argument in the OP seems to be that the character should not have taken it, which leads me to ask “why did the DM put it there?”
I will, however, argue that, in any gaming session, some level of metagaming is inevitable. If the DM mentions something, the players will inherently think that is important. If the DM sets up some quest, the players will think it is one they can actually survive and beat. And if the DM makes a point to mention an item, the player is going to think it needs to be taken, and is foreshadowing what they may face in the future.
Any actual answers to the challenge are what have already been stated. Well, plus one more: “If a smithy is making silver daggers for people, there is likely something around here that requires a silver weapon to kill. And I may face that something in my mission.”
As you said in your cough, dubious. No evidence for it. The evidence is that there is an emergency at hand, lives possibly in danger. You equip up and go. Leave a note.