Unexpected power ON of ES700

[MarkHJ]

I have an ES700, approximately five years old, supporting a desktop PC. It does not run continuously and is shut down over night. In the last week or so it has started unexpectedly powering ON, so by morning the battery is almost dead and the UPS is giving an occasional warning sound. 

A few experiments confirm that when it is "in the mood", it will randomly power on - I've switched it off, turned the mains power off, and suddenly minutes later the alarm starts sounding for running on battery. It does not seem to be a battery fault (self-test is OK) and it is not the cat powering it on (not a joke, we have several ES700 and 400, and have added a safety cover over the switches on account of the cat), so I am wondering if it is a fault and I need to replace, or whether I have accidentally engaged or disabled something. 

The only other observation I have is that this has happened in the wake of Storm Darragh (power out about 36hours). This particular UPS was running at the time and did get its battery completely discharged, but that was over a month ago.