CliffordK

For decades, the high-end bikes have been made with really thin tubing. They may look solid, but it doesn't take much of a whack to dent them. Still, it doesn't just happen with ordinary every day riding. And the demand for light bikes will maintain the demand for thin tubes.

The problem here is that the OP discovered a dent in his bike. And, it is a pretty significant dent. He doesn't remember denting it. So the next step is to remember when it was last seen without the dent.

It is possible the dent occurred while riding. Gunshot, rock kicked up by a car, etc... but unlikely. And one would think someone would notice if one's bike is hit by a rock or something.

So, the next option is that the dent happened prior to the last ride, and the OP just noticed it after the ride. Yes, the OP says he did a cleaning or cursory inspection, but it is easy to miss something that one isn't expecting.

So, the most likely thing that happened was that the bike fell over on something prior to the last ride.

Perhaps a co-worker accidentally knocked the bike over, and picked it up without thinking anything more of it, but the fated dent was there, unnoticed.