Oh, Excel also has comments which sound like what you are describing, except
they do not have anything to do with formulas (Insert-Comment).

"Michael Gula" wrote:

Cells in Quattro Pro containing a formula display a mark in the lower
left-hand corner. Holding the cursor over that mark causes a pop-up to
reveal the formula. (They are called formula markers.)

I thought Excel had a similar feature but, if it does, I haven't found
the way to turn it on.

Does it?