Hello,
My friend and I are pretty proficient in Excel, but have never seen this
before. When selecting a cell, a yellow box pops up with information. I can
move the box around the sheet, but it does not appear to be a comment. (When
I do add a comment in the sheet, I can right click the comment to change
properties and it also has a line attaching it to the cell it's commenting on
- this yellow box does not).

I've tried searching help, etc but don't know enough of what to look for.
Any help would be appreciated!

I'll send you a screenshot if you need more info - that speaks more than
words!