Why do I see abandoned bike locks at the bike stops? Presumably the bike has been nicked but the lock has been left behind 
Looks like a job for Adrian Monk.
Anyone got a theory?
Locks can be heavy and awkward to carry around on a bicycle.
Far easier to attatch it to your regular parking post and leave it there for your daily commute.
Or the bike tube/frame was hacksawed.
I hadn’t thought of that, but I don’t think it’s the answer. Some of the abandoned locks are quite modest and I see the same ones week after week and they’re never actually securing a bike. Also if you used this wheeze you’d have to leave a lock at every stop you used.
So the thief gets two halves of a bike?
Monk: But…the locks are all different. They don’t match!
Stottlemeyer: They belong to different owners. Of course they don’t match!
Monk: But it’s not even. They should match!
Stottlemeyer:Do you think they all get together and decide on what type of lock to get?
Monk: It would be more uniform.
Stottlemeyer: Peoople don’t care about uniform! They use whatever lock they have, or whatever’s cheapest! Can you focus on what’s important here?
Monk: But it’s not right. They should match.
Theory: The lock was used to lock the ends of a cable which is passed through the bike and rack, and the cable was cut.
I experience a poignant amusement when I see a lone wheel locked to a rack.
Oh, you do, do you? Well let me tell you, IT’S NOT FUNNY! Why yes, it did happen to me recently, why do you ask? 
Lots of locks are left at commuter bike racks, and the same bikes are locked there everyday. So, yes, it happens a lot. I see it all over Boston/Cambridge.
I also see bikes abandoned but still locked in a few places. Two in particular have been there for over a year, some parts have been taken, but they’re mostly intact.
A decent Trek bike stood locked on State street & 2nd south in Salt Lake City for 2 years. This was right downtown where there is reliable police presence so I’m guessing that nobody dared come by with a pair of bolt cutters or hacksaw and take the bike.
Reminds me of the old story about a university campus that had no problem with theft, but did have a problem with bikes being parked illegally and blocking footpaths, etc. The university came up with a system where bikes were impounded, and the offender had to go and see the groundskeeper and pay a small fine in order to redeem the bike. A smartarse student, sick of paying fines, decided to park his bike illegally, and put a lock on it so it couldn’t be impounded. He also put a note on it: “Take my bike now, ASSHOLE!” When he came back later to collect his bike, there were TWO locks on it, and another note: “Pay the fine, ASSHOLE!”
Seriously, I think it is usually regular commuters. I leave my padlock and cable at work most days I ride, because I won’t be riding anywhere else. I bring it home on weekends, in case I need the bike to go shopping, etc.
Oh, sorry. Well, I did say poignant.
It’s a pain but when I lock my bike I remove the front wheel (quick-release hubs), and pass the lock (a Kryptonite) through both wheels and the frame. Not just the spokes of the wheels, the rims.
Of course, it’s a only a matter of time before someone shows up with a Bic pen and picks my lock.
People still ride bikes? Come on!
My GoB from Arrested Development.
Just thought I’d pop in to say that this reminded me of the story “Or All the Seas With Oysters” by Avram Davidson.
You know they’ll replace that for free, right? Check their website.
You get a new pen? 
Some of the components on high end bikes are expensive, even if the frame gets ruined the theif gets all the other parts. A bike should be locked around the frame and both wheels to protect the most expensive parts and make it most difficult to steal.
Wouldn’t it be easier to remove those components on the spot, rather than cutting the frame in half? High-end components are particularly easy to remove because they are designed for ease of maintenance.
Yes and no. Quick-release “skewers” let wheels come off in a couple seconds, but suspension components are not so quickly disassembled.
In theory, someone could also re-weld the frame where they cut it.
I do remember seeing a device that prevented the seat (post) from being removed, marketed as a solution to ‘seat theft’. So you are at least partially right.
However, as gotpasswords said, while the wheels come off quickly the rest of the parts take some fiddling wih an allen wrench to get off and stuff like the fork and shifting mechinisms would take a bit of time to get off. Most bike tubes are very thin, I would think you could go through them with a hacksaw in a very short time. You only need to go through one tube then flex it enough to get by the lock. Then all the components are conveniently on wheels for the thief.
I don’t know how incognito one could be when clipping a cable or hacksawing a frame, but I’d guess that waltzing off with a wheel or two and some gearshifters would be less easy to conceal. At least with using cablecutters you could always explain that you dropped the key down into the sewer but you really really needed your bike.