I had a similar issue - really frustrating because I wondered whether the battery was due a charge, so checked (via the Garmin headunit, not connecting to the PC interface) and it reported 50%. Went to go for a ride the next day and it was completely flat.
I’ve only got one bike with EW-WU111. The bike was in a stand, so the switches were not leaning on anything.
My best guess was that I had my Garmin headunit on charge, so it was switched on (not sure why they switch themselves on when charging…) and may have connected to the Di2 and drained it.
Otherwise some Di2 bug that will be difficult to recreate and track down as it seems so infrequent.