[AngelicaDeAlquezar]

Remember that in Spanish we need the double negative:
- Se suponía que no había nadie allí.


I'd propose:
- Se suponía que no debía haber nadie allí.


I used "deber" because if you say "se suponía que no había nadie allí", you are saying nobody thought there was anyone there, but maybe there was. With "deber", there is a reason why people shouldn't be in that place.


When you say "se suponía que nadie estaría ahí", you mean nobody was going to be there, but someone actually considered going and/or went.


Also, the place of "no" is important. If you use it before "suponer", then you are saying that you don't assume something.
- No supuse que estarías aquí. -> I didn't assume you'd be here.
- No se suponía que cobraran la entrada. -> I didn't expect they would charge the admission.


Your proposal "se suponía que nadie debía estar ahí" is correct, although it implies the idea that people were forbidden to be there, while "no debía haber nadie ahí" is for me that they could choose, but most of them preferred not to be there.