Steve just told those of us in the control room that the custodian who goes into the IFO room regularly steps on the blue support beams to reach the top of the chambers to clean them.  Since we have seen in the past that stepping on the blue tubes can give the tables a bit of a kick, this could help explain some of the drift, particularly if it was mostly coming from TT2.  The custodian has promised Steve that he won't step on the blue beams anymore.

This doesn't explain any of the ~1 hour timescale drift that we see in the afternoons/evenings, so that's still mysterious.