I had some strange behavior with my battery last night...

It was at 50% and I needed to have it full today, so I plugged it on my computer, and turned it off (the computer).
The USB hub where the N9 was plugged has its own power source.

Yet, this morning I was awaken by some blips from my N9: the battery was depleted

My N9 is maybe 2 months old, and I didn't really had any real issues with the battery before.
The only application running was "Battery usage", as I always have it in the background.

Is it something people have already experienced ?