Let me start off by saying I know little about plumbing but am pretty handy.

My problem: We have had a bad smell in our unfinished basement for years since I bought our spec house. I have never been able to track it. We have two sewer drain lines that go down in the floor: one smaller one coming down from a main floor bathroom, and the other 'main line' coming from everything else on the upper levels. As part of my troubleshooting, I inspected all clean out connections and all seemed tight. All had some sort of sealant on the threads except for the main sewer line horizontal clean. I noticed also noticed that it looks like at some point it might have had a few drops on the floor under it. It doesn't have the typical pipe wrench plug, but it looks more like something that a biscuit joint piece would fit into to turn.

I taped a plastic bag over the cleanout to see what would happen. The next day my basement smelled better. I did notice that although I had squeezed all the ‘air’ out of the bag, it was fully inflated which makes me suspect that gas is coming out of that cleanout hole even with the plug in there.

1. Is there supposed to be some sort of ‘lock tight’ on that plug to prevent gas from excaping?
2. How do I fix this?

Thank you!