That's a real possibility, although I think they would have heard or felt the impact. An explanation might be that it might not require that large an object to severely damage the tile. I believe that the orbiter normally flies with cargo bay doors open to radiate heat to outer space and wouldn't present much of a silhouette to debris in earth orbits. NASA should have good probability data for such an occurrence and it would be interesting to see how it stacks up for the 107 and some flights. I don't remember what the overall probabilities are for Shuttle Missions but I think we are pretty close with the two losses. Bottom line however may really be are we lucky or unlucky.
My theory was that there may have been a failure in the left landing gear door seal that would have the effect of a hot gas jet penetrating the interior wing structure and would result in it's failure. In today's conference, they said data on the tires just dropped out and then said later that the wheelwell compartment DID NOT see an increase in temperature, so that could rule out a seal failure. I'm eager to see if any of the new "unreliable" data they expect to recover may still hold this possibility open, if there was in fact a temperature increase.