My very first instinct was mice or rats in walls/floors. Now that's just a complete guess, but was the first thing to come to mind.
In my experience you can barely hear them in the day but you can hear them scratching or moving when it's quiet.
I hope it's something else though, like maybe a new chicken rotisserie opened up in town and the dog smells it. Could be lots of reasons.
Hopefully this rings a bell for other members.