Ahhh...
Third stealth outing with the F3 and the first time I forgot to bring some tape to secure the hold switch

The inevitable did indeed happen and I ended up missing the final song of the night. Thankfully it was just a slightly lacklustre Travelling Wilburys tribute act that I was only taping to keep my hand in.
I had the recorder in my bag and it looks like the switch must have been knocked when I stood up (the band asked everyone to stand while they took a photo of themselves from the stage...) I initially thought that the batteries had died but when I switched it back on I still had two bars left.
Lesson very much learned - I'm going to keep a small strip of gaffa permanently stuck to the back of the F3 so it's always to hand.