The honest answer
Until you do something about it, yes, a lost key can start your car if the wrong person finds it and knows which vehicle it belongs to. The car has no way to know the key left your possession. It still recognizes that key as trusted.
This matters most if the key was lost with something that identifies your car or address, such as a wallet, a registration, or a keychain tag. In that situation the missing key is a real risk, not just an inconvenience.
How to shut a lost key out
The fix is to erase the immobilizer's memory and relearn only the keys you still hold. Once that is done, the lost key is electronically dead and will not start the engine even if someone finds it and it fits the locks.
If you have no keys left at all, we perform an all-keys-lost job and program fresh keys, which removes the old ones from the system in the process. Either way, you end up with a car only your keys can start.
What to do right now
If your key is missing and you are worried, call us and we will come to you in West Palm Beach to wipe and relearn the keys. It is the same kind of step you would take with stolen house keys, just for your car.
It is also smart to keep your VIN and registration separate from your keys going forward, so a lost key never points a stranger straight to your vehicle.
Still have a key or fob question?
Tell us your year, make, and model and we will confirm exactly what your vehicle needs before we head out.