Even if you do it like there are some enforcers that "spy" on their own patrols, PCs could avoid it by ambushing the hidden one first (as its role implies other goblins don't know it's there) - as long as the ambush is two stealth strikers hitting in surprise round or some clever silencing tactics, single monster shouldn't be too hard to put down in surprise round.
You can't give goblins too much credit for intelligence or (in case of the spy) dillingence in their duties...if you do it that way, that spy should be some other, more lawful race...