They do not. All creatures start with a rather large range of Detect Hidden, but they dont passively detect. They only detect if their combatant suddenly hides or goes invisible. Controlled or not, this seems to be their behavior. This will require some changes to AI but shouldn't be too difficult. I'll keep the passive detected hidden in, but make the default to false, for custom monsters, etc.
And this is based off tests I did after we spoke, Dan.