It seems that there could be an issue with the Modular Cars. A few people on Survival EU are reporting losing access to their vehicles even with the keys in inventory. After removing and reinstalling module and crafting a whole new cockpit module still unable to access. Resulting in having to destroy the vehicle and source a new chassis to be able to use again.
This has happened to myself and 2 other people on the server in last 2 days. 1 Person stated that it happened to both the vehicles currently in their possession. All cars invloved also had the locks installed but user had keys in inventory