Purchased a older home which has a toilet and sink in stalled in a closet we do not use it though there is a foul smell coming from it, where can that be coming from and what is it?

Toilets and sinks must be used several times a year so the water in the trap does not evaporate and let sewer gas into the home.Use both fixtures and if the smell doesn't leave you may need to replace the wax ring under the toilet