I noticed a few tracks I have downloaded individually will already have my best time listed. At first I assumed that because it was one I played online that there would be an autosave replay for it. After trying to find it it is not there when I select the autosave folder in game. What does the game use to record the best time in these instances?

You're looking for where the game automatically saves your best times, that's the autosave folder. It wouldn't be anywhere else unless you manually saved it.

Some versions of TM2 ago it was not saving PBs. So some older PBs online will not have accompanying autosaves. It's working now though, so you should have them for any newly set records.

Eyebo, the tracks I am seeing my previous best time without ever saving a replay or having an autosave replay are indeed tracks I drove during the beta.

I was just curious as to where the game would store those times.