<Here is a question that has been bugging me about the theory that the insulation striking the belly of the wing caused this accident. If so, why has NASA not found any tiles around the launch site? Was the Shuttle over the ocean when this happened and so any tiles knocked off landed in the water. Would such loose tiles had been seen in the close-up of video of the insulation hitting the wing? I see vaporized insulation, but nothing that looks like tile coming off.

The insulation hit the wing about 80 seconds after liftoff. The Shuttle was doing about Mach 2 and was well over the ocean at the time.

It's possible that the tiles wouldn't be knocked off immediately on impact but perhaps weakened. 80 seconds after liftoff is roughly 20 seconds past Max Q (maximum aerodynamic pressure), but the pressure would still be substancial. It's conceivable that the tiles were damaged at impact and further weakened during the ascent. They might've fallen off on ascent or early in the reentry phase. An Australian astronomer in California reported witnessing what could be early signs of tiles falling off when the Shuttle was near the Bay Area. If so, it'd be very hard to find any of the tile fragments. While they wouldn't burn up, they'd likely break into very small pieces and be scattered by the wind. News reports indicated that some very light pieces of debris were still falling some 6 hours after the breakup.