What does it mean when someone leaves a white towel, tee-shirt or plastic bag in the window of a car they have left on the shoulder of the highway?

Maybe it’s a regional thing but I see it all the time. A car is abandoned on the side of the highway and someone has put a tee-shirt, plastic bag, etc. always white, rolled up in the drivers’ side window so that most of it is hanging from the outside. Have you seen this? What does it mean? Is the driver indicating that he has not just abandoned his car but is coming back? It doesn’t seem like it really matters because your car will be towed after a certain period of time whether you left a sign or not? And where do people learn to do these things? I never got the memo.