How do they know?

One of my boarding dogs, a Golden retriever, Knows which is her unit!!

I watched with fascination as Tilly would come out into the big run, exercise and do what she needed to do and then go to a unit and bark, obviously wishing to go back in and who can blame her as it is freezing outdoors and they all have underground heating on. What fascinates me is that of the 15 units, which all look the same, she went to hers. All items inside are from hear i.e. carpet and bedding so how did she know exactly which one she came out of?

Dogs are soooo clever.