Confirm the procedure before the day
Ask where the car should be left, whether a specific bay or level is required, and where the keys go. Getting this wrong can leave you liable for the car until it is found.
Check also whether an out-of-hours return fee applies — it is charged separately from the pickup fee.
Photograph more, not less
With no staff to confirm the condition, the photographs replace the inspection. Use the flash, take every panel and wheel, the interior, the fuel gauge and the odometer.
Add a wide shot showing the car in the correct bay with any location signage visible, so the time and place are established.
Liability until it is checked
Until the supplier inspects the car, responsibility for it usually remains with you under the contract. Anything that happens in the car park overnight can be attributed to the rental.
This is why the timestamped photographs matter: they establish the condition at the moment you left it.
Keys and confirmation
Use the designated key box or drop slot, never leaving keys in the car or with a third party. Photograph the keys going into the box.
Expect the return confirmation by email the following working day, and chase it if it does not arrive.