Users in Grand Theft Auto V have reported vehicles disappearing from their in-house car garage.
Users in Grand Theft Auto V have reported vehicles disappearing from their in-house car garage. Reports suggest that car upgrades have been lost as a result, but a solution is in the works. Rockstar has commented on the bug stating that "this issue may occur if you drive another character’s personal vehicle. We are looking into a fix to prevent this from occurring in the future. If you would like to receive an automatic email notification when the fix is live, please log into the Support Site and click Subscribe at the top of this page. In the meantime, please try to avoid parking a car in a garage, driving one already in a garage, or getting in a default vehicle, when playing as a character other than the one with your upgrades."
Source: Rockstar
Until then a PC version is still ...