Just askin' . Any chance the " dog sitter" could be involved in this. Do you usually take the dog's collar off......why did she. How did your dog lose the collar.
Other than the obvious questions......it's rather freaky......but kinda cool too.
I'm betting that somebody found the collar, had a lab that they wanted to be rid of, put your collar on her so that whoever found he would call YOU. Good move on their part; after all, YOU are going to end up keeping her. BETCHA!