..tlogs are telemetry logs, they only apear when you have some kinda of radio link. logs are dataflash logs.
So my first reaction was, "but I have lots of .tlogs and I've never had any radio link!"
Then I figured it out, and it explains why none of my .tlogs contain actual flying. Occasionally I arm the copter with no battery while it's plugged in to see what the motor outputs are in the Mission Planner. Because the copter is armed and linked, it makes a .tlog.
Big d'oh for me! Thanks Michael!
I guess it's time to set up that xbee pair I bought way back when...