Stick a magnet to the inside areas of the bed where the tape runs.If it doesn't stick in some areas I would suspect the previous owner used the tape as a backer for body filler.
Also inspect the back of the cab for any scuffing or slight dents. The bed may have been contacting it and rubbing due to misalignment and that was a quick fix.
Only other thing I can think of is the bed was off for painting and someone put tape there to protect it while standing the bed on end on a couple 2x4's.
Archimedes screwed us !
I knew a blonde dyslexic agnostic insomniac - she'd lay awake at night, wondering if there really was a Dog.
2003 Ranger Edge 3.0 manual 2WD