I might throw in the info I've found about this:
I am semi-active on the official forums, and we have had a long conversation just about this. From our attempts, there are no links between game completion and the finds. There seemes to be a randomizer somewhere in the mix, along with playtime and possibly driven miles. I have a save where I have done no more than the starting events to choose between the STi and evo, and other than that, just driven around, to see if they pop up. Unfortunatley I havent driven that long yet, so no conclusive theory has come yet.