Apparently some countries place a UV or invisible ink stamp on a visa page before affixing the visa sticker.

Found out the hard way when I thought I was being clever and removed a
visa sticker because the passport was running out of pages. The passport
control officer wanted to know why the passport was tempered with. He
placed the passport under a blacklight box and showed me the big stamp
that said "CAUTION: XYZ VISA REMOVED/TEMPERED" ("XYZ" being the
name of the country that issued the visa) The sticker came off real easy
and left no residue. Well.... now I know why... they didn't have to use
superglue because they stamped the warning on the blank page first. ^

Anybody knows how to take a picture of this so I can share it here? I
tried it with a regular UV flashlight, but my digital camera didn't pick
it up.

Since adding pages became $$$$$, got the passport renewed instead.