My sister came to stay for a weekend last year. She's tactful, so she didn't say anything until the second evening, when she finally mentioned it gently. "It smells like dog in here. Not bad, but... you must have gotten used to it." I hadn't gotten used to it. I'd gone completely nose-blind to it. What felt like a normal house to me was immediately noticeable to everyone who walked in.
I'd been spraying things, running candles, washing the pet's bed, vacuuming twice a week. None of that registered on my end because I couldn't smell the problem anymore. But the smell hadn't gone anywhere. It had soaked into the furniture, the carpet padding, and the walls, and every warm day was pulling it back into the air. I just wasn't the one noticing.
If you have a pet, there's a real chance this is your house too. The people most affected by pet odor are almost never the people who live with pets.