… is there any other explanation for it besides drugs? The kid has no interest in science, btw.
Has (s)he ever taken a science class? The teachers usually mention how expensive they are, and that they’re a BIG DEAL, and don’t screw them up! My only other guess is that (s)he might have been like “ooh expensive and fits in my backpack!”
From my drug experiences, to need a set of balances for drugs means that (s)he’s past the getting pre-weighed bags from someone else and getting a bulk discount to split hirself.
Science teacher here. Come June, my mentor always reminds me that we need to lock the balances in cabinets that the janitors won’t have keys to. Not the we are worried about the adult janitors, but the school hires kids for extra labor. Years back (prolly ~10, and before my time), some unlocked balances were stolen, and a kid was suspect (although nothing could be proven). The assumption is that they were stolen for drug selling.
ANY other explanation? Sure. They look cool and sort of creepy. Someone found one in the dumpster. Uncle Fester left it to her in his will. There could be a thousand reasons. Are any of them more likely than the drug hypothesis? I have no idea. That would depend a whole lot on the kid in question and whether or not her other behavior points to drug use and/or dealing.
But I will add that I’ve never known a drug dealer to use a balance, including the few dealers I know who own them for other purposes. They’re just not convenient to lug out and tare properly when your buddies are hanging out sharing a joint and buying some while they’re in your living room. There’s a little tiny hanging scale* that looks a lot like a compass (the circle-drawing kind, not the North locating kind) which fits in your pocket or stash box and weighs out 1/8ths and 1/4s very well. That’s what everyone I know uses. Retails for $1 at the office supply store, instead of the hundreds you can spend on a balance.
I suppose if one were moving kilos of drugs, a larger scale would be in order, but even digitals are cheaper than quality balances these days.
*Like this.
It’s enough to warrant a talking-to, especially since it counts as paraphernalia in most jurisdictions. Since it is your home, that puts you in an uncomfortable place, especially if dealing is involved.
In your situation, I’d confiscate the item and release it only if a good explanation for its use was offered. This would also be an opening to examine other aspects of the kid’s personal life.
Drugs or stolen? Both require you to do something. Call up the parents if you get the so and so gave it to me bit.
Male kid or female kid?
How’s the kid’s eating?
Maybe he/she is weighing food as part of an eating disorder?
If the kid isn’t interested in science at all, and has no obvious hobbies that might account for some scales, there aren’t many other explanations that don’t merit a discussion.
It’s almost certain that your son/daughter is not only a scientist, but a mad scientist.
My condolences.
Have you seen any sharks or henchmen about?
I would have to say it would depend on the kid.
That being said, I’d be mighty worried - MrMoto has a good plan - confiscate them, and get an explanation.
Have you considered asking the teenager if there’s any other explanation?
I used to use those balances to measure out pyro charges when I was doing that as part of my job.
Still an item of concern that needs to be addressed with the teen, but at least it’s not drugs! 
No other plausible explanation at all.
Not necessarily true.
When I was a teen, I had a scale I used for weighing out chemicals for making homemade bombs - purely for the fun of setting 'em off mind you. I quit that hobby before I blew myself up or parts of myself off.
Maybe he/she wants to be a supreme court justice?
Or an alchemist?
Or, as others have said, is fascinated with the look of the things?
I have to say, that if I was weighing drugs or explosives, or for that matter doing anything illicit with a pair of scales, I would not leave them lying about. Did you go digging for them? Or were they just out there?
Talk to the kid, don’t be afraid of the answer and listen to what they have to say. If it’s drugs tell them you do not want them in your house [unless of course you want them to share] and be fair yet firm.
Ahh…perfect worlds sound so nice.
I found them under the bed. I also just now found a stash of what looks like tobacco. Are there any drugs that look like tobacco? I doesn’t smell like tobacco, but I have very limited experience in these matters.
Er, “looks like tobacco”?
Clearly a stash of oregano.
Or other herbal remedy. Hemp’s an herb, right?
Really I’d be pretty concerned. I don’t know what marijuana smells like (only a vague memory of what it smells like when burning, as I never used the stuff, and I was never close enough to non-burning MJ to know what it was like) but I think some discussion is in order 
When I was a young teen, probably 13 or 14, I thought science gizmos were cool, even though I wasn’t into science. A particularly old balance would have been pretty cool on my shelf next to the surveying thing-a-ma-gig I had. I wasn’t into surveying either. I also had a rusted out old maritime-looking compass and a jewellers loupe.
I would just ask the kid what the deal is before jumping to conclusions.
Ah, the details make this look less kosher.
MJ has a very unique smell, but if you’re not familiar with it, it’s tough to describe. It certainly smells nothing like tobacco. Bear in mind that tobacco may be flavored/scented, but I don’t know why you’d be weighing it.
Well, looks like an uncomfortable conversation for you. I don’t envy you.
At least you have not used the substance yourself! I will have a hard ethical time when I catch my kid with dope.