Perhaps he was checking out your cart to see if you were loading up on things that indicate you will be eating your shabbos and yom tov meals home, alone, and was thinking of extensing an invitation.
Perhaps his Rav said to throw out his iopd, blackberry and cell phone, leaving him bored with little else to do, so he decided to play a game called spot everything that starts with the letter M in that guys cart.
Perhaps he was really bored, but also very shy and wouldn’t strike up a conversation and was hoping you might give him shalom and strike up a conversation and instead was left wondering why you were looking at him with a negative expression on your face.