The location where my car was parked was remote. There is no explanation for the proximity of his idling vehicle.

The remote parts of the car park are usually where the least desirable sorts hang out. (I park in the same areas due to an oversized vehicle, so notice them.)

I also think you're being paranoid.

So you paid by cash and as a result home depot secretly called the cops on you, and the cops knew where you were parked and sent a car to passive aggressively harass you by parking nearby?

Maybe the cop was parked out of the way eating his lunch.