Have you checked the boot and the base of the rear seat?

The rear firewall doesn’t usually seal against the rear seat pan and fuel leaking from one of the fuel line joins can sneak through into the seat pan where it is absorbed by the rear seat base foam. BTDT.

If this is happening the whole area (car inside/outside/garage) will stink of fuel.

With the cap on fuel will not evaporate even after years of storage (BTDT too).

Cheers, Ian

PS have you checked the engine oil level?