[Jefferson Coastal]

I do a fairly thorough clean of my steel fermenter after fermentation.  Scrub it out, soak in hot PBW, rinse in hot water, sanitize with StarSan.  Looks good and smells good. 
Then I put it away (in a clean, dry pantry closet).  After x weeks, I will take it out for brew day.  When I pop the lid and smell, typically a sour beer smell.  So I always do the same cleaning process on an already clean fermenter the same day of brew day.

I've never had infected batches, nor off flavors in my beers, as I'm fairly consistently able to turn out good to excellent batches.

My question is - Is that smell normal?   Should I replace the lid gasket?  At this point doesn't seem to affect the final product, it just bugs me.

[Semper Sitientem]

It’s a characteristic of silicon seals - they retain odors. It’s annoying, but it’s not going to affect what’s in your fermenter as long as you keep to your cleaning regiment. I’ve tried to soak them in a 50/50 vinegar water mixture for several days and it does reduce the odor but will never completely go away. Buying new seals can get expensive.

[BrewBama]

After cleaning, I’ve found air drying HomeBrew equipment with valves open, hoses hanging, fermenter upside down, etc helps. (Don’t forget to close valves for brew day.)