You are out for the day with a group of English friends, who have also brought along some other people that you don't know very well. In casual conversation you ask one of your own friends about one of these other people, and he tells you ~ almost offhandedly ~ 'Oh, Steve? He's all right really, but sometimes I wonder whether he isn't a few sandwiches short of a picnic.'
What is this supposed to mean?
Steve very rarely contributes as generously as everyone else when eating together; i.e., he's mean ('stingy', even).
Steve is absent-minded, didn't make or bring along his own packed lunch, and is sure to be 'on the scrounge' for other people's leftovers when you all stop to eat.
Steve is not very intelligent.
Steve is a trainee caterer who has not quite finished his course yet.