Sounds weird, but it could be non-sketchy. Is there anything remarkable about the car? Cult classic? Usual body style? Rare color? Anything like that?

Usual Unusual body style

It sounds like a variation on casing the neighborhood to me, but it could be legit. Is there a Neighborhood Watch in place? This is the sort of thing that NW was started for.

The car is a very plain '98 Maxima. Unsure of NW existence. I'll ask the neighbors.

A word of caution:

This happened to a friend of mine a couple years ago. His apartment complex was being painted and one of the guys from the work crew approached him in the parking lot and asked if he was willing to sell his car (which he wasn't). The guy also left a phone number (which my friend didn't keep). About a week later, my friend's car was stolen from that same parking lot. He's not 100% sure the two events were related, but he has his suspicions.

aaaah, I tend to leave notes on cars and scooters (vespas) when I was was a much younger dreamer with my full name, address and phonenumber stating "ossom wheels if you ever want to sell, please call me!"

But these were all classics in some way, like a champagne colored Volvo p1800 ES or something heart stopping like that, and since I figured nobody would ever sell, it was my way of giving them "yes your wheels are gorgeous" compliment. I still two thumb up when good wheels pass me on the street (though it looks even sillier now that I have to let go of a pram to do it).

* funny thing, I made friends with a Vespa owner years later who talked about "some nutter leaving notes on every scooter I ever owned", and yeah, that was me.

This used to happen to me all the time, actually. I had a 94 explorer and I spent a lot of time at my house so it was parked outside alot during the day. Every couple months pretty regularly for 5 or 6 years some mexican dude would ring my bell and ask me if I wanted to sell it. Sometimes they left phone numbers, sometimes they didn't. I never really thought there was anything suspicious about it and it was never stolen, so I dunno.

The Spousal Unit called the cops and they are cruising the neighborhood to see if the guy is still around. They apparently think something is up.

Hmm, seem to recall a neighbor mentioning a rash of break-ins a few years ago.....

The first time I ever sold a car, it was because the meter reader knocked on the door and asked my wife if it was for sale. It was my old '72 Dodge Dart and it needed engine work at the time; it barely ran.

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I ended up selling it to him.

I heart that photo, Doohickie.

It's the only one I have of that car. This was in the days before digital cameras and stuff, and heck, at the time it was just an old car that got me from Point A to Point B.

I was thinking about what type of counter-measures to put in place but the Spousal Unit stopped me.

"If it gets stolen, we get a new car," she said.

That's my girl.

I've had a garbage collector ask if I wanted to sell a car that was sitting in our front yard. I sold it to him. Sometimes people are seeking a particular car for various reasons (my garbo was looking for a particular model "S pack" Falcon (pretty much like the one below) because he was a big fan of that model).
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While it is possible that there is something nefarious going on here, it's also possible that it's entirely innocent. Did you try calling the phone number, just to see who answers?