Is there a word for seeing without being seen? I want to write about those on the advantageous side of tinted glasses. Visual/optical eavesdropping is the closest phrase I can think of.
Try covert, as in covert surveillance, or in your example, a covert observer.
I'd probably go with spying:
to watch secretly usually for hostile purposes
Does incognito work for your purposes?
: with one's identity concealed
No single word comes to mind, but, if this for journalism, a short phrase might be as good as, or better than, a single word (which would probably be some latinate obscurity). How about discreetly observing/watching/gazing/...?
With reflective lenses, you can reflect on the world that you're reflecting back, gazing discreetly while dressing chicly, grabbing a eager eyeful without risking an irate earful.
"covertly spy on"
"voyeur" - implies fetishness or at least and wierdo
Of course there's invisible.
Out of the field of vision of a person. Covered by another object. In the dark. That is perfectly camouflaged.