Given the frequency and consistency of narrative with which it appears in the press, at this stage I'm convinced it's an empty UAV used entirely for some marketing purpose
EDIT: And, in fact, the one publicly-released payload I'm finding was a test of a new heat pipe.
https://www.youtube.com/watch?v=j56s46e97Lo
Based on what this talk indicates, the US Government is doing some questionable(...) things in terms of signals intelligence in space.
We probably won't know what X37-B is doing in space for years but I imagine they will have at least tried to do some big brother exercises. The US/Western Intelligence community effectively treats all communications as fair game - they always have (e.g. MI5 were capturing encryption keys from neutral embassies using microphones in the ~50/60s) they just possess the means to do it efficiently at large scale now.
I imagine a large part of its mission is also just testing military equipment that can't go on the ISS but that's not as sexy.