Why would someone steal my rear derailleur and hanger?
Some thieves will intentionally disable a bike so that it can’t be ridden home. Then they’ll come later in the day or evening with the appropriate tools to finish the job. 

Often they’ll steal a wheel or your seat or handlebars. But if you had those secured with security bolts then the derailleur might be the next easiest thing to steal that would totally disable your bike — and on many bikes you just need a hex wrench to take it off. And you can put it back on just as easily when you’ve taken the rest of the bike. 

Otherwise, no clue. It may have been done by the same thieves who stole an inner tube from a bike here on SE while leaving the tire and rim.