Have your crawlspace checked?
I've been called to many houses for a complaint of a smell. This could be from a toilet that is set incorrectly or a fixture/sink that hasn't been used in a while, however, it could be much worse. I was called to a job today to an older home that had a smell. We ended up finding four broken drains in the crawlspace after the main drain going through the space had fallen down. Not only were the drains broken, the main sewer was backed up from roots in the line so every bit of water that went down the toilet or other drain was going straight into the crawlspace. Talk about a mess!