Yup.
Is she hot?
Oh, you people are no fun:
Put on along rob with a hoodie. Get a scythe. Now go out on your front porch and point directly at your crazy friend’s house.
She’ll close her blinds.
Are you wearing pants in this story? Because that sort of will change my recommendations.
I’m going with Shakes on this one. (No, not really.)
She sounds a bit nutty. Avoid her, including any “innocent” conversations. No conversations are innocent to her. Anything she asks, if you feel you must respond, simply say, “I hadn’t noticed.” Don’t admit to anything, don’t ask anything, don’t volunteer any information.
And remember the rule: Don’t put it in the crazy.
The Rule is: Don’t stick your dick in crazy. If you’re gonna quote it, get it right. 
Not wanting to enable the probably-wacky person too much, but would it be possible for you to hang some roll-up bamboo blinds or something like that on your porch so you can’t see her window from your usual smoking spot? That would probably wipe out most of your view and reason for being out there, but she would then not be able to see you on your porch, and if the cops come by again and want to talk to you, you can point it out as an attempt on your part to stay out of her sight.
And I agree, in part, with Shakes–you passed up a great Halloween opportunity.
At no point in my entire life have I invoked this clause as advice and regretted it.
It seems like you could change the orientation of the chair on your porch to point to another direction.
Turn your OP (and any significant omissions) into a chronological listing of your significant interactions, with exact dates where possible, and approximate dates where not. You may not want to initiate anything, but you want to be ready if she does. Keep a copy on your computer at the office, and a paper copy in a locked desk drawer. Keep another paper copy within easy reach of your front door.
If the cops do another are-they-monitoring-me-or-not stop in front of your house, go out there, pleasantly introduce yourself, and ask them if they’re just catching up on their paperwork, or if there’s a reason why they chose your house in particular to stop in front of. If they ask you why you asked, give them the 2-minute summary of what’s been going on, and hand them the list. Tell them that just in case she has complained to the cops about you, here’s the sequence of events as they unfolded for you. Invite them up on your porch so they can see why it’s natural to look in the general direction of her house, and remind them that you had no idea you could see her house until she made a point of telling you.
At work, you may want to take the initiative with HR. (I don’t think anyone can tell you you should or shouldn’t at this point; that’s really your call.) If you do, explain that there’s this co-worker you didn’t even know until a few months ago, who has taken the initiative at every step in your acquaintanceship (this part’s important - make sure they hear it), in a way that’s feeling increasingly creepy to you. Tell them you aren’t looking for any action on their part, you’re just meeting with them so that they know there’s this situation, and that they will already know about it if it escalates. And then show them the list and go over it with them.
If you don’t take the initiative with HR, at least you’ll have the list handy in case anything further happens. If she gets creepier at work, you should definitely take it to HR then. If she complains about you to HR (that possibility is really the main reason why I’d suggest going to them now), you’ll at least be prepared to give your side.
I just wanted to say that cops stop on side streets all the time to fill out paperwork, talk on the radio, etc. I really, really, really don’t think it had anything to do with you. If she’d called about you, they would have gotten out and spoken to you. Forget about the cop car.
However, she sounds like a total nut, and I would avoid her as much as you can without setting her off in some way (she might get weirder if she feels slighted). DO NOT make stalking jokes anymore. Document the things she’s said, with dates and times. Document any further conversations with her. If she gets worse, take your documentation to HR. If she goes to HR, you’ll be ready.
Consider moving.
ETA: RTFirefly said it better 
but that post wasn’t there when I started
Except I disagree about going to the cops. I just don’t think that had anything to do with you at all, and I think approaching them will make you seem overly paranoid. Unless you work in some magical town where cops are known for their understanding and professionalism, I would avoid involving them unless absolutely necessary. It will make you seem a tad nutty to assume they are there about you, and with the documentation it will look worse (in my opinion).
I think once might be happenstance, but given that the OP smokes a lot on his front porch and hadn’t seen the cops stop in front of his house before, a second time probably wouldn’t be coincidence.
I would continue going about my life, including smoking on my front porch. I’d be civil but otherwise do nothing to engage her in conversation. Oh, and I’d lock my doors in case this turns into a scene out of some psycho-horror movie.
This. I know a cop that would spend his entire shifts parking his car on side streets in different areas so he could basically screw off… then near the end of shift, he’d take the car out and run it up/down the highway to put the proper number of miles on the car so it looked like he’d been patrolling all night.