Is there a visual equivalent of the word "overhear"?
I think glimpse is close to the meaning you are looking for:
- (tr) to catch sight of briefly or momentarily.
(AHD)
Espy
https://en.wiktionary.org/wiki/espy
"To catch sight of; to perceive with the eyes; to discover, as a distant object partly concealed, or not obvious to notice; to see at a glance; to discern unexpectedly"
Overheard implies not directed at or intended for the hearer. Espy should carry the same implication. Standing behind someone staring at the computer screen is not espying, but opening the door and seeing something on their computer screen is espying.