Another Parking Lot question

It seems like we've had a lot of questions lately about parking lots... specifically Walmart parking lots. well, here's another one...

I was at Walmart yesterday. Had to pick up the basics: milk, bread, canned green beans...

So, i was loading up the car afterwards. Taking care of erica. Yes, i put the cart back where it belonged. As i was put my groceries in the car, another car parked next to me. A little old lady and guy stepped out and oggled over Erica. (This happens quite often.) Then, they went inside.

I continued doing my business. Then, i see this shady-looking guy come walking up. Mid-twenties, white, shaved head, ratty green jacket. Hmm... I was a bit scared (I always imagine the worst-- like, what if he's gonna kidnap erica). But i said to myself: Its daylight out, and tons of people are around! But still, i kept my eye on his every move.

He walked up to the car next to me... the one that was just parked. He takes out his keys, and pretends to try to unlock it. I was watching carefully-- he didnt put the keys IN the lock, just kinda pretended to be. I think he realized that i was watching. So, he took a few steps away, pulled out a cell phone, and pretended to be talking to someone.

i wasnt sure what was going on. i KNEW it wasnt his car. but was he gonna steal it? or what? I hem and haw, fiddling with stuff in my purse, trying to think what to do...

~Do i yell to him: Thats not your car! It belongs to a little old lady and man!
~Drive away like nothing happened?
~Go inside the store and report what i'm seeing?

Well, i pull out of my spot. As soon as i start doing so, he puts away the cell phone. I drive to the other side of the parking lot, then drive back to see if the car is still there and if he's trying to get in it. As i drive by again, the car is there but he is not...

So...
?What was going on?
?What should i have done?