You get those cars from the Signature Edition of the game. The following cars are included:
Audi R10 TDI Stealth Model
Honda NSX GT500 Stealth Model
Mazda 787B Stealth Model
McLaren F1 Stealth Model
Mercedes-Benz SLS AMG Stealth Model
Nissan GT-R GT500 Stealth Model
These cars are not tradable and are not rare, so to speak, as anyone who has bought the Signature Edition of the game will have them. As for how you got them, I have no idea unless you have the Signature Edition and maybe created a second account, in which case they will show up that way I believe.
You say that you just logged into your PSN that you haven't used in awhile. Do you mean account or different PS3 altogether. I asked because you have been here trading cars and whatnot recently which leads me to believe you have been logged in recently.
Again, a little confused by your posting. Maybe if you give a few more details, we could help with this mystery of yours. I don't think PD just randomly sent them to you unless you do indeed have the Signature Edition.
EDIT: Seems you just got the game according to your first post, which further puzzles me with your "haven't logged in in ages post" When you got the game, was it indeed the Signature Edition and if so, did you redeem any codes. That would explain your mystery. As I said before, these cars just don't randomly appear.
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