The baffled woman, who had been attending a religious service in Adenau near Koblenz in Rhineland, immediately drove the dachshund to a nearby police station, officers said in a statement on Thursday.
But while officers were inspecting the animal, another distressed woman came in to report her beloved dog as missing.
The mystery was eventually solved. It turned out the mix up had occurred when the women had mistaken each other's identical cars for their own - which they had coincidentally parked next to each other.
"Obviously, in this rare case, the keys also fit in both cars' locks," the police said.
The dog owner had got into the wrong car and left without realising her mistake. Then the other woman had returned to find the dog on the back seat.