Some households have a grimy smell? What causes it?

[Bronzeacorn]

When I was a teenager I dated a guy and his home house (he lived with his mum and dad) was just grimy, not even particularly messy or cluttered... just badly maintained and never truly clean. Think brown rings on bath/sink grime on every surface. Their house had a smell I can't explain.... its not like a sick to your stomach smell, no animals in the house so not cat wee it was bearable but noticeable.... i can only describe it as greasy?? But I don't even know if that'd the right description.
I recently went into.a neighbours house and I noticed the same smell for the first time in 20 years. There is no judgment here I was just interested if other people have noticed this and what causes the smell? Do the homeowners get it? Could my house smell like that and I dont know? Just looking for others experiences I suppose out of curiosity

I worry about my house smelling. I properly clean at least once a week but have cats and I think pets add to household smell, as well as cooking (and I do cook proper meals). I have old carpets because I'm waiting to get new flooring. I'm not the most domestic and worry about it! I wish I was one of those 'love housework' types because I'd love an always tidy and clean home.

Cats, dogs and damp.

Biggest thing that gets me.

Even dogs that supposedly don't smell. Nope they do.