My house is well/septic. I have lived there for about a year. It is a three story house (which includes an unfinished basement). There is a shower on the main floor that has not been used since we moved in. Yesterday, I came downstairs from the 3rd floor after having taken a shower to discover that the main floor reeked of sewage. I tracked it down to the unused shower in the guest bathroom. This has never happened since we moved in. Any idea what is causing this?

Calm, with an unused drain, the water in the trap will evaporate with time. Slowly pour a pail of water in the smelly drain to refill trap and stop sewer gases.

If the drain will remain unused you can add cooking oil to the trap as it doesn't evaporate.
Good luck

Ahh... I get it!. I'm surprised that it would have taken a whole year to evaporate though... I'll give it a try. Thanks for your prompt response. I am curious though, how do I know when I need to have the septic pumped (short of having raw sewage backing up into the house)... is there some indication?. or is it just time based?

Calm, when to pump has a bunch of variables, like what size tank, how many people are using it. Do you not have an access lid? Maybe see how full of solids it is with a long stick. Or just have it pumped every few years to be sure. Good luck