If there is a single mouse in a house I can smell it straight away it's hard to describe, it's like musty, when I went to visit my gran, as a boy, she had mice, sometimes she'd suddenly say shush, keep still, & a mouse would be right there in the hearth three feet away from where we were sitting, it'd stop, sit up & clean it's whiskers then scurry off, it was a very old cottage, there was no electricity only a big oil lamp on the table, she lived on her own & said they were company for her.