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?