Whenever you are entering the buildings, you must be very cautious. If you see the doors opened, then it means someone is already expecting you. In one case the other guy might have looted the building already and is not waiting on its prey (you). On the other hand, he is already long gone, and you are entering there just to waste your time. In any case, you must make sure that someone is in there. You must then take all necessary measures to take the enemy out. You might hear a lot of sounds if he is moving up and down the buildings looking for loot. This can give you valuable information about how many enemies are currently hiding in the building and if going inside is really worth it.