Grand Theft Auto Online – How to Fix Joining GTA V Online Error

I think what you're describing is where you replace one expensive car with another in your first garage. This means that your previous car's tracker would have been removed and it wouldn't have been available from your garage... If this is what you're experiencing you mightn't be able to get your car back.

The only way I know to solve this is to re-buy your car and store it in your other garage and to make sure that it is going to your other garage before you store it. Otherwise I cannot help. But that is my opinion.

Based on what you've described, it is likely that the online lobby you were in had a hacker/modder. These people are able to do a lot of crazy stuff, including spawning weapons that you can pick up.

These weapons that you've picked up should not persist between sessions, so re-starting the game or entering a new lobby should remove them from your inventory.

Rockstar has, thus far, been largely ineffective at stopping many of these hackers. But if you manage to capture video proof of someone hacking (often difficult unless they're being obvious), you should submit it to Rockstar support by opening a new support request.