Just did a search for previous report of damage which might be relevant and found two.
STS-27 Mission report:
Initial post-flight inspections of the exterior surface of the Orbiter revealed significant tile damage with 298 damage sites greater than 1 inch in area, and a total of 707 damage sites on the lower surface of the vehicle. The area of major damage was concentrated outboard of a line from the bi-pod attachment to the external tank (ET) liquid oxygen umbilical. One tile was missing on the right side slightly forward of the L-band antenna. Also, there were many damage sites consisting of long narrow streaks with deep gouges.
NASA Reference Publication 1390:
(STS-45) The Atlantis suffered 2 gouges on the upper portion of the right wing leading edge. The most probable was a low-velocity (relative to the spacecraft) debris impact on orbit or during re-entry. However, Johnson Space Centre engineering has not ruled out prelaunch or ascent debris as the cause of the damage. This particular event raised concern about the consequences of a higher energy impact to the integrity of the spacecraft.