Ok, here's a weird one. I have a guitar track that is one of two routed into an aux track acting as a subgroup, and that aux track then goes into a bus. What I'm seeing is the output of the aux track is 6dB higher than the output of the guitar track going into it. This is still the case when the track is soloed so it's not some other track feeding in as well, and there are no other sends from the guitar track. The aux track's gain and volume are both 0. No plugins on the aux track, no ProChannel.
I fixed the problem by just creating a fresh aux track and routing the guitars through there instead. I can't see any difference between the new one and the old one but now I get unity gain when I pipe audio through. So, I don't really have a problem that needs solving any more, but I thought this was worth mentioning. Maybe someone else can reproduce it, or maybe someone knows a hidden setting that I accidentally managed to switch. 6dB is a suspiciously 'round' number, as if the signal was being routed in twice by accident.