Pretty much everyone. The way I read it in YW it's Roll=4df+Stealth+other modifiers. If Roll > difficulty, you didn't completely fail to be sneaky. Anyone in a position to see you rolls 4df+Alertness/Investigation/whatever+modifiers and has Roll as their difficulty to beat to detect you.
Standard contested skill mechanics.
Actually, Lots of folks in the thread have said to do the same thing. So "essentially everyone" is a bit much.
Now, I can see how one could parse the Hiding trapping as saying something else. Namely, roll to hide vs. a difficulty + modifiers. Then use the roll as a the base difficulty for the finder's roll. But, there are problems with this reading:
1. Ignores the text on Stealth which says Stealth is always directly opposed by alertness.
2. Ignores the established mechanics of contested rolls, which fit the situation perfectly. The hider is either defending, blocking or maneuvering. Why set a static difficulty? What good is it to succeed at hiding and then fail at remaining hid in the next roll?
3. Requires special rules to make it work at all.
4. Ignores the commonsense notion that if you're hiding in an empty room nothing of import happens until the guy looking for you rolls.
But hey...