I do not see how it makes sense to actively be informed about being examined by someone.
And even if it did, there should not be any difference between being observed and being examined by the examine tool.
In conclusion, if there were a message to appear whenever someone is looking at you (Be that with the intention of spotting a disguise, guessing your skills, keeping an eye on the surrounding people in general or just looking in the direction of another character) this message should appear in every single one of those cases.
Assuming there is a way to implement this mechanically, it would mean a lot of messages.
Most of my characters are either curious or paranoid, which means that if you are standing in my characters sight, you will get this message from me ... every few seconds!
Standing in a busy place like the outskirts or mist camp would therefore lead to several "you are being observed messages" per second...
It would not make sense to only get this message when the examine tool is used and not in those other cases where someone is observing a character.