On some cars the control units do not go into sleep mode after the car is switched off, and the battery goes flat within days. The owner typically replaces the battery, and then the fault returns, because the battery is not the problem. The search starts with measuring the quiescent current, and the labour makes up the bigger part of the bill.
What to check
- Ask specifically: has the car ever stood for a few days and then not started
- Check whether a battery was replaced, and if so, how many times
- After locking it, watch how long it takes for the onboard systems to go to sleep