Now, let's talk about curtains. How often do you think curtains in rental flats are washed? Yeah, about as often as those hotel bedspreads; actually, probably far less often because curtains are not usually a place where spilled drinks and food, vomit, pee, or semen tend to land. Unless those curtains get really disgusting (full of cigarette smoke, for example), they probably hang around without getting a wash for quite a long while.
And while they are hanging around, they are touched quite often.....open, close, open, close, open, close. So, if you want to find a location in a rental unit that might still have a lot of DNA from a number of people, you just might choose the curtains.
After, you test them, you would then have to run down everyone who ever stayed in the unit (and their friends who visited) and all the employees who might have entered the room in order to exclude anyone who have had reason to be there. You could then try to match the DNA to known criminals if you have their DNA or their DNA is in a data bank.
Of course, in the end, if you still have some DNA that can't be matched to anyone, you can always suspect that mysterious DNA be the calling card of some unknown person who committed some terrible crime that happened in that location.
Criminal Profiler Pat Brown