FWIW I have two Kobo Touch's one running the old firmware and one that has the new (Glo) firmware. I have seen similar battery problems with both units running in airplane mode and with sleep set to 15 min and power-off set to 30 min. In my case when the problem occurs the Touch does not transition from sleep into full power-off as it should but instead stays in sleep mode and consumes a disproportionate amount of power in a short space of time (as if the processor was stuck running at 100% in the background).
I'm currently exploring whether the sleep/power-off interval is the cause in this thread: https://www.mobileread.com/forums/sho...d.php?t=204803
if you would care to contribute your times?