Why (and how) would someone steal only my 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.