Learn English – Word for seeing someone’s moment of realization

Try covert, as in covert surveillance, or in your example, a covert observer.

The only actual Latin word that means ‘all-seeing’ which I can find is omnituens, which is frightfully rare and poetic even in Latin, and doesn’t seem to have been used more than a small handful of times in English—half of them being people telling others that there’s no such word.

I would not advise using that.

Then there’s omnipercipient, which is at least an English word, though still not a common one. It clearly means ‘all-perceiving’, but that’s quite close to ‘all-seeing’, and the OED defines it as “seeing or perceiving everything”.

All in all, though, I think you’d be better off with Greek than Latin here. The Ancient Greeks had tons of verbs meaning ‘see’ in various shades, and a few were combined with the prefix pan(t)- ‘all’, the Greek counterpart to Latin omni-. Thus, for instance, πανόπτης panóptēs means ‘all-seeing’ and has actually come down to us in English as panoptic, which has two separate meanings: ^OED

1. All-seeing; (fig.) comprehensive, covering every aspect of a subject, all-encompassing.
2. Of the nature of or relating to a panopticon; in which all can be seen.

In addition, there is pantoscopic, which is most commonly used as a synonym for bifocal (describing glasses and lenses), but also has a more generic meaning: ^OED

3. All-seeing; having a wide range of vision or coverage.