From what I can tell, it's ads from visiting girls that can overstay their welcome. Seems like visitor ads stick around for 30 days while local ads disappear at midnight.
Good news is ads are listed according to time submitted, so once you have gone down to the ads posted at 12:01 am, anything further back is one of those lingering ads. I generally just ignore them--if a visiting lady is still in the area, she usually refreshes her ad everyday.
Also, when you open an ad, it gives the date submitted at the top, so you can check how old it is.
