Alcohol is hydroscopic (absorbs moisture from the air). Your 99% will
go down to 91% if you leave the bottle open. Fortunately, it takes
some time so you don't need to be fanatical about replacing the cover.
https://en.wikipedia.org/wiki/Azeotrope
Note that 70% alcohol is what you want for a disinfectant because some
water is needed to kill of hydrophylic bugs.



Well, good to know. Also that 91% is good enough for this job. Stuff I wouldn't even think to think about, turns out to be very relevant.

I tried to find and clean the mirror. No success getting in far enough.

Now it often reads when first loaded. But has trouble after it's loaded, and stopped, on finding it to read again. Can't imagine what causes that?