Advice for 15C for junk on display

[everettr]

I have two HP 21S calculators that had large dark spots on their displays. In 2014, I kept one (Singapore 3010S04039) on my desk at work, where they never seem to turn off the bright fluorescent lights. Over the course of two months, the dark spot on the work calculator faded away, not yet to return.
For the at home calculator, I started storing it on a shelf under a skylight. Over the course of a year, the original spot slowly faded away. However, a smaller new squiggle began to appear.
Hypothesis: Bright light without IR fades spots, but bright light with IR component fades a spot while growing new ones.