Stealthers in close proximity..

[Jack]

With a high detect hidden skill you will start to reveal stealthers when they move. This requires at least 80 detect and has a 60% chance of passively revealing someone when they stealth (move) within 1 tile of you.

This information was taken from this stratics page:
http://web.archive.org/web/1999100909194...detect.htm
"Having a high Detect Hidden skill helps to automagically reveal people that stealth in your vincinity."

However the exact mechanics of it were guessed by me.