Very interesting note on page 119 of the 1e DMG:
"Various devices for seeing at a distance (such as crystal balls, various scrying devices such as dishes or pans of water or mirrors, and spells such as clairaudience) are DETECTABLE."
The astral nature of remote viewing
The explanation for this potential detectability is the fact that they scrying device is a form of assisted astral travel. The astral spirit of the remote viewer actually goes to the location being viewed. That is why someone viewing a scrying device goes into a trance while viewing, as their spirit is actually undergoing astral travel, their conscious mind cannot be with their physical body.
Countering the Scrying Attempt
If the creature being observed is a magical creature/spell user/psionic, it has a change of realising that he is being observed.
Detection of Invisibility Table, pg.59 of the DMG
As per the DMG, if the creature realizes he is being observed, he can cast a darkness or a dispel magic upon the spot of observation (described in the DMG as a "small disturbance in the air caused by the magical viewing"), which is the location of the remote viewer's astral body.
Vulnerability to astral attack
If the creature being observed, or anyone at the remote viewing location, has psionic powers or the ability to see into the astral plane, the remote viewer would also subject to psionic attack. Attempts at "soul capture", such as a magic jar, could be conceivable as well....