Something like this happened on our flight back from MIA last month. On our descent into London (seat-belt signs on) the automated emergency announcement came over the PA system and oxygen masks dropped from the panels.

We were on the upper deck of the 747 and worryingly (I think) not all the masks drop; from memory, it was only those above Row 62 (we were row 64). I don't know if they dropped in other cabins.

The pilot quickly came on the PA and said he didn't quite know what had caused it, but it looked like some sort of computer glitch (isn't it always?) and there was no emergency. We landed safely.

I'd forgotten all about that until i read this post.