We have an intermittent problem with sewage odor in the house. Doesn't seem to be realted to any specific use of bathrooms or clothes washer. There is no bubbling sound from any of the bathrooms. Recently had septic tank cleaned but odor was occurring prior to draining the tank. Vent pipes do not appear to be clogged. We snaked them without finding any obstruction.

If your traps are full of water and doing their job properly, you might have to caulk around your toilet bowl base. Sometimes methane gases can come from there as it is after the water seal in the bowl. I have encountered this problem before and seales it up, eliminating the problem.