returning a rental can in Venice on a Sunday

Beats me about Venice, but it's fairly standard everywhere else in the world. One thing I do if the clock will tick over into another day of charge (rates are charged by the twenty-four hour increment from pick-up, not calendar days) while the keys are in the box with the office closed is to hook a note to the keys noting time of drop. Tying it on with dental floss works pretty well. I've only been unbelieved/ignored once, I think.

If you are concerned about key drop at a closed office, you could return the car at the airport instead. At least one company ( Hertz) encourages you to do that by imposing a 24€ surcharge for dropping at Piazalle Roma. That helps cover the cost of transport into Venice.