Maybe the agent did make a mistake, they have a large fleet of cars to deal with. But it seems unlikely. Surely if you ended up collecting keys from a desk, they would have given you the little receipt thing, and I think fuel type is on there.

I'm struggling to work out how you could be driving a car around for nearly a week and still be unaware what fuel it takes.

What was the car? It sounds like there were about 7 of you, so an MPV most likely, which are rarely bought as petrol, especially in Germany.

The last two cars from Sixt I have had, have had model badging which gives it away (120d / TFSI). And they have both had little repeater labels in the fuel flap.