There was also a piece of foam that got loose during the STS112 launch.
Pictures were taken of the ET, right after ET separation; same for this one like they normally do.
There were no problems, then.

I'm afraid we might never know exactly; it could have been a debris in space, hitting the low wing side of the Shuttle. I wonder if that could have been noticed (noise) or if the astronauts were in a too noisy environment to notice it.

The foam is really too lightweight; but if it hit the leading edge of the left wing, I don't know.
Those tiles on the leading edges are the toughest ones because they get the full heat during re-entry.

Raoul