I have a Garmin GPS that flattens Ni-MH 2700 mAH AA batteries relatively quickly even when it is switched off (half the charge lost in a month). I found that if the batteries are removed then they hold their charge pretty well. I now insert the batteries just when I need them and am now a happy bunny!
I think the problem is a 2200 microFarad electrolytic capacitor that is connected across the batteries having a higher than I would like leakage current (all such capacitors leak electricity). It is simpler to remove the batteries than get the capacitor replaced with one that is hopefully better.