I am pretty sure that if you try to book such a rental, either the quoted price will include the extra day (until Monday morning) irrespective of when you bring the car on Sunday, or it will say "location closed at the time you selected, please try to change the time).
You can easily verify that by trying different return times for your booking to see the quotes.
If my version is correct, you book the car till Monday morning and return it at any time you want on Sunday.

The only place I know to quote Sunday returns with the office closed is Hertz in downtown Cologne (Germany) - and they have gated parking garage with a time stamp on the entry ticket.