It sounds like she is casing the neighborhood.
I am serious. You have a woman you have never seen, you suspect drug use, and she is just randomly in the area? And you saw her three times now?
I'll bet she is looking at cars in the parking lot. Maybe she is giving the information back to her dealer, or, possibly even, pimp.
I used to live somewhere where a certain woman, very much like your description, and older, was super friendly with everyone.. but also had a way of seeing what they could get out of you. Then, when you said "no," everyone started to sneer and treat you different.
This one is trouble. If it is a police decoy, call up the station, ask for a supervisor, and ASK, and share the concern. That seems a little different than ratting her out.
I used to live somewhere with a police force so strict that this person would have been approached by an officer, asked for ID, and a chat about why she was there, that they got calls about her being suspicious. How it went from there is up to them. And if the has drugs, and she ain't a cop (what would constitute an undercover being out there? It does not add up, unless there was a recent rape case or something, maybe,) is up to them.. and if she has those drugs, or is on it, she gets to sober up in jail, then get out and do it again.
If you feel like something isn't right, and you have some inner voice telling you it doesn't add up, ask the police chief if there are decoys. Same way we have campus police around here. They look for ANYTHING like this.