Why (and how) would someone steal only my innertube? Upon my return from winter holidays, I found that someone had stolen the rear innertube from my locked bike. They took only the innertube: the rear wheel was still there, the chain correctly attached, and the bike otherwise seems untouched (so far at least, for obvious reasons I haven't ridden it yet).

I had the front tire and the frame locked to a bike rack.
The bike is stored indoors in a common bike room for my apartment building, so access is somewhat restricted to those with a key. 

Why (and how) would someone steal just the innertube?
Occam's razor says that you should try the simpler/null hypothesis first, in this case that your tube wasn't stolen.  There are two alternate hypotheses that you should test:

1) Are you sure that you had one in the first place? Perhaps you were running tubeless and never knew it?

2) Are you sure the tube is actually gone? One thing that can happen (either by prank or accident) especially with schrader valves if you don't have a valve cap (but also with presta if you don't have a locknut) is that when the tube deflates, the valve can retreat back into the rim and get lost (I'll avoid making any dirty jokes about this).

I grabbed the following image off an unrelated page but you can imagine what would happen if the valve got pressed in even further and got lodged inside the rim. It would look like there was never any inner tube in the first place.



I've almost done this (gotten the valve lost inside the rim) trying to inflate a tire. It's not hard and it could even conceivably happen as a tire self-deflated.