Whether you own the vehicle or if it's unowned, it'll send you several messages saying it's not yours when you can still put a part on.

This is going on the Backburner until I get a chance to fix the vehicle interaction code. Right now it's very messy and barely held together to not crash. Fixing that code will be a significant time sink, and I really don't want to do that right now.

Fixed in the interaction overhaul that was done in V17.2.0.