Septic tank or sub pump riser near their home?

Oir finished basement has started leaking recently and I am trying to figure out where it’s coming from. In my search, I noticed a green lid next to our house that says “fatal poison gas” which I read is usually a riser for septic tank. I was under the impression that our house was on the city sewer system. But our house was heavily remodeled about 10 years ago so I would think they would have removed this at that time if not needed.

I took a look at our home inspection to see what the inspector said and he noted it could possibly be a sub pump. Wouldn’t we have noticed a plug somewhere if we had a sub pump?