We have a 1930s house that we took down to the studs 10 years ago and put back together. Last winter we had a "fishy" smell in a little entry that sticks out of the house. I had to hang heavy blankets to try to keep the smell out of the rest of the house. Sometimes the smell comes back for a while. It is very strong, almost burns the nose. Doesn't seem to matter on the weather, can be hot/humid or rainy or dead of winter.
Now another "fishy" smell came last night at top of stairs. I can't quite put my nose on it, where it's coming from. It is very strong & offensive. Makes me want to move out! We have all hardwood floors. All old has been gutted out except between stairways going up & down stairs, this would be still the old lathe and plaster and possibly any old droppings that could've gotten between there. (When we took old walls apart they were filled 3-ft high with animal droppings and misc they had carried in.)
We have had lots of bats in our attic. I've heard that bats have a really bad smell. Don't know how to tell if it's something like this or where it's coming from.