The other day, I pulled off the road, key off and out. I then noticed the passenger side tail light was on ? Disconnected the wire, went out, put the wire back on, lights on ? I since have been unable to reproduce this ? Any knowledge ? Help ?

If you leave the turn signal lever on it will do that - Marker light or something

Make sure your turn signal switch is in neutral

Ah ha ! that's probably it ! Thanks 2Rider914 and @eric9144

It was apparently a 'feature' for parking, I think the idea is that if you're parked parallel on a street you could illuminate the outside (road side) light for visibility.

So the corner of your car would be seen when parked on narrow European streets

Everyone here has scratched their head and wondered why half the car is lit up one time or another - Welcome to the club.

Rumor is that BMW's would do the same, but so far no driver has used their signal to activate it.

Must be the same for Mercedes-Benz

Another old fart left the turn signal on.