Why is it when I open my bathroom window for air, the result afterwards is always a certain distinct smell?

This has always been the case in homes I lived in, or if I’m staying over in other family members. It’s happening in the home I living in now, and the smell is reminding me of homes I grew up in or when I stayed over as a child at my aunt or uncles homes. These are detached homes or rowhomes.

I’m not sure how to describe the smell. It’s not a foul odor but it’s so distinct that it’s always from the bathroom when the window is opened for air and I just want to know what it is, how to prevent it, or if I should be concerned.

Thank you!

Edit: I think the best I can describe it is maybe like a stale air kind of smell with a hint of must. I’m wondering if it is a normal thing, since I’ve noticed the same smell in a number of bathrooms in different homes.