The left landing gear door is that rectangular patch just at the inflection point of the wing. It looks close to where the plume appeared in the video, so the "door damage" theory seems plausible.
Note, BTW, that the video itself shows no obvious tile damage. This says two things:
1. The NASA folks who evaluated this made a reasonable decision.
2. The calls for ground cameras or sat-to-shuttle views wouldn't have seen anything.