Not obvious, not placarded and not spelt out in the brief. (CSD was shocked, took my details and passed them on to BA, and very rapidly I had a nice letter, and the brief in WTP was slightly modified, as were the seat back cards).
I too have found that my life jacket wasn't where the brief said it was.
I forget the airline but I remember once the brief mentioning three different places the life jacket might be depending on what class you were in. It took me a moment to think where mine might be...and it took me a while to actually find it. If I hadn't found it prior to take off and we had had an emergency, in the dark and in the panic it would have taken me ages to find it, if at all.
The other thing (a tip I picked up on here actually) I do is to count the number of seats to the nearest exit during the brief.