This little light of mine, driver side marker/turn signal illuminated

Walk into the light my son.

 

Never saw that.

 

Does it have a automatic light setting on the headlight switch?
If you have a battery disconnect switch flip it off for a few minutes.
I don’t really mess with anything paccar much, but it’s likely a module that didn’t shutdown properly.

 

Are wires even connected to what they are going to anymore? Fuse boxes don't count, relays don't count.
The answer is probably NO, because everything seemingly goes through a CPU at some point.
Every little thing is controlled by a computer.

Bits and bytes, zero's and one's.
One zero not turned to a one for some random reason... there is your stray light.

So yes, you are reading too much into this.

 

It’s a PACCAR and there is a computer module for everything. That light is controlled by the Body Control Module (BCM). You’re going to have more random stuff coming your way, until the BCM is replaced. Most likely, your shop will do nothing, because replacement is the only sure way to diagnose a failing module. If the truck starts and runs, no one else but you cares. And, PACCAR sucks!