Taking something that belongs to someone else is, beyond a shadow of a doubt, stealing. If an item that was personally owned goes missing and ends up in the possession of another person, they stole it. This applies to cars, televisions, and nearly everything else a troop may own. But, for some reason, few in the military bat an eye if the missing item was issued without a serial number — it happens far too often in the military.
There are a few old sayings in the military about this very concept. "Gear adrift is a gift." "There's only one thief in the military — everyone else is just getting their stuff back." And, of course, "it's not stealing if it's tactically acquired." Meaning that if you see something left — you can take it. After all, standard-issued gear is so widely distributed that it's hard to prove who the gear was issued to originally.
Take a medium-regular service uniform (the most average-sized service uniform that's also the most sold-out at any Exchange) for instance. If someone takes it out of a washing machine and it doesn't have the original owner's name sewn onto it, it's almost impossible to confront the person who took it. If they aren't caught in the act, they get away with it.
Eventually, troops will have to turn in their gear to the Central Issuing Facility (CIF) before PCS/ETSing. If a troop is missing a piece of gear that must be returned, instead of taking the hit on the chin with integrity, person A jacks person B's gear so they don't end up with a hefty charge. Remember, "there's only one thief in the military," and so the cycle continues. Person B then needs to decide between eating the fine person A should have faced or pass the burden on to person C, who has conveniently left their gear unattended.
There is, however, a third option most troops don't consider: reporting it to the MPs. If you can prove that your gear was taken and there are signs of forced entry (broken locks, broken doors/windows, water on the ground of the washing machine), you can easily take the report to CIF and explain the situation.
...Or you could just sweet talk the supply NCO. That works, too.