Grand Theft Auto 5 developer Rockstar Games has cautioned players from using the game's garages until a bug can be fixed.
As detailed on the developer's support forums, there's a glitch that sometimes causes cars to lose their upgrades or outright disappear from the safe houses in which they're parked.
"We have received some reports about problems with cars and garages, particularly after upgrading and then completing a mission, and are looking into this now," explained Rockstar. "Unfortunately, we do not currently have a workaround to recover a car if it is missing."
While Rockstar is still investigating the cause of this issue, the developer stated that it thinks this may occur if you drive another character's personal vehicle. "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."
Rockstar also made sure to note that cars parked in one character's garage will not appear in another character's safe house. That part's not a glitch, but simply a design decision.