In this case they had recieved fire from the house, or the immediate area. It was a firefight. The house was quiet when their advance got close, but they still envisioned terrorists in the house. They shot around openings and used grenades. They also thought fire came from the second house. That may have come from the 4th house where they had an AK at hand, or the other terrorists that retreated.
Later the Marines ended up in a fighfight that ended with a Harrier's ordnance. That was about 2 blks away. It may have been with some of the same terrorists that started this fight.
Interesting. I didn't know that. Just shows that the entire area was still 'hot', and the Marines would have been remiss NOT to have been wary of every house.