Gerard, hygros can go flaky, too. I have one in my winador that's driving me nuts. I just put batteries in it and recalibrated it and it's still off by a mile. I'm going to throw it in the calibration bag again and see what it says (I use a bunch of 65% Boveda bags in a gallon ziplock to calibrate).
Being as your situation doesn't make sense, I'm just taking a wild shot. It wouldn't hurt to replace the battery if it's been in there more than a year.
Your humi could have split a seam or something, too.
You know what you're doing, so I won't bore you to death, I just wanted to point out that hygros can get goofy cause I'm going through that right now.
