A few nights ago I noticed my 10.10.2 rMBP had woke up from sleep shortly after I had put it to sleep.
From the Console app (in /Applications/Utilities) I found:
11/21/14 2:19:50.000 AM kernel[0]: Wake reason: XHC1
According to a quick Google search, XHC1 is BT-related. So for now I'm assuming it was one of my peripherals causing a wake for some reason. OP, I might recommend doing the same thinggo ahead and wait until this issue crops up again, and when it does, check Console and see if you can find a message like this. Might help