for the last three evenings we have had a very strong sewage smell only at night. It does not seem tied to water usage. For instance, on Monday I ran the washer/dishwasher/showers a lot and the smell was there, but last evening we were out to dinner and we entered the house and the smell was there again. It disappears during the day. It is not localized to a bathroom or sink but is in a general odor in the lower floor of my home and basement only. We have an old home -- and have had the outside drain and main sewage line cleaned out about 4 months ago (we had a distinct smell in the basement that was consistent and increasing over time-- we called in an emergency service who cleaned the interior back up).
We are attached to the city sewer system.

ok smells are some of the toughest plumbing problems to diagnose, firstly you need to start with the main culprits : floor drains, and other fixtures usually in a basement that get little or no use. pour water in your floor drains, run water in sinks that don't get use and flush any toilets that may never get used, especially in the summer time these can dry out extra fast due to ac usage.

Most often, the smells that appear to go away don't usually disappear, but your body blocks out the smell as unnecessary information and ignores it. If you leave the house and come back, the brain tells you again, but will then file the information away and ignore it. I am guessing that you may have a missing cleanout plug in the side of a floor drain that is your culprit. grab a screwdriver and a flashlight and pop the covers on your floor drains. There should be a hole in the bottom, full of water, and a plug in the hole on the side of the drain. If the water or the plug is missing, that is where your sewer smell is coming from.