A1. You are by yourself and you see a complete stranger buying food and
for whatever reason you feel like muttering "That person is buying
food over there".
zero particle; あの人、あそこで食べ物買ってる (if you don't specify him with あの, it's が; あそこで人が食べ物を買ってる)
A2. You are with a friend who also sees the stranger and also doesn't know
who the stranger is and ...