Like: " Mr SO and SO, do you happen to know where the wonderful porcelain salt and pepper shakers have gone? They seem to have lept off your tray for some reason... maybe on the floor somewhere? Can you check? Or else I might have to get into your seat and check around.. I wouldn't want to find them in maybe the seat pocket, or maybe even in someone else's bag, you know... "
Well I would say that this is a little too condescending/accusatory, but a tactful FA would simply say "Would you mind checking if you could see if the shakers accidentally fell on the ground as I couldn't find them on the tray. Let me know if you find them."