Red Shoe hanging from neighbors front door knob?! What does this mean?

So I live in a townhouse, and my neighbor is a young woman about 25. She lives alone. I don't know for sure if she works, because I never really see her come or go at any normal intervals. Every once in awhile, she will hang a red high heeled shoe (kind of like an ornament on a string) from her front door knob. It will hang their for about a week or so, and then it disappears for awhile. What does this mean?