The GM can gain a surprise bonus of +1 if a Sentry is present, or other specialist creatures as noted (such as a Skaven Assassin).
If the Leader scores equal to or higher than the GM, the monsters have been surprised. The Leader can then move each monster one square. Now start a combat turn.
If the GM scores higher, the monsters surprise the Heroes. In this case, play then proceeds to the GM phase of the first combat turn (in other words, the Heroes miss their first turn!).
Having nearly completed their Quest, the Heroes discover a group of Skaven (including a Sentry) behind a door. The Leader and the GM each roll a dice (the Elf can see into the room so the Heroes add a total of +2 and there is a Sentry present so the GM adds +1); the Leader gets 7 and the GM 4, so the Skaven are surprised. The Leader may then move each Skaven model one square. Obviously, he will look to set them up in such a way that gives the Heroes an advantage. A normal combat turn will now begin. Note the door on the far side. If the GM chooses to have the Sentry open this door during the GM phase, whatever lies beyond must be generated at the end of the phase. If this is a Lair or Quest Room, with more Skaven, heavy reinforcements could soon be forthcoming...
As you can see, in the Enhanced - Advanced Heroquest version of the rules, surprise heavily favors the Heroes but the GM will always place the monsters. This is more realistic, especially in pre-designed dungeon sections, and also more balanced.
Be sure to check out the DOWNLOADS page of this blog for the full set of rules, though each of these modifications can be used alone or in any combination.