Toilets, Sinks, Showers, Tubs and Disposals - Sewer Smell

Since last week I noticed a sewer smell in my basement. I checked to make sure the trap lid was closed and it was. I removed it to make sure there was no clog and it was clear. I flushed all the toilets and ran all the sinks and faucets and showers in the hose to make sure the water was draining. Every thing worked fine and there was no back up. The smell doesn’t seem to be coming from the trap. It smells as if it is coming from the center of the room. I am stumped and beginning to get concerned. I was hoping if I ignore it it will go away but I know that isn’t going to happen. I live in a single family detached home so the smell is not comming from another residence. Any ideas.

Sometimes I've seen the perimeter (rain) drainage run under a house where a drain is set in the basement floor (flood insurance?), then on to the city's storm drain system, seperate from the sanitary system. Finish the basement and that hole gets lost.

Call city engineering, ask if they've made any changes to the local sewers lately.