Robert, Yes I understood this but it started from Gerry's "This is a little standing water in the toilet but it is not always enough to make a good seal." I assumed he meant a toilet with tank.
Gerry, It could be clogging in the main and the waste doesn't flush away. But that has nothing you can do about it. I had something like that here. But I pull a little weight here being Mayor of the block. lol
I keep all my drains closed for the same reason. There is a large rubber type shower drain cover made that might help? Most places even dollar stores have them here. Maybe you can find it there? These gases (besides the smell) can be dangerous, in health and in flammability.