First off, that map is infamous in my gaming group. It's from "Keep on the Shadowfell," an adventure published before even the 4e PHB. As such, it's got a lot of... quirks, and then the Stealth rules changed in the PHB2 (see Sage's link at the bottom for full details of current rules). The party is supposed to enter from the West (left) and so the kobolds are hiding on the wrong side of the bushes. There are other problems with it too, as we'll see.
I'm using the DDI Compendium glossary for reference:
- Yes, Stealth is an active check against passive Perception.
- They should not get any bonuses to Stealth checks. Total cover and superior concealment allow Stealth checks, and that's enough.
- You need total concealment or superior cover to make a Stealth check. The boulders provide superior cover, but the bushes in that encounter "are lightly obscured and provide normal cover," which means that technically the kobolds not next to rocks shouldn't be able to hide at all.
Keep out of Sight: If the creature no longer has any cover or
concealment from a target, it doesn’t remain hidden from the target.
The creature doesn’t need superior cover, total concealment, or to
stay outside line of sight, but it at least needs partial cover or
partial concealment from a target to remain hidden. A hidden creature
can’t use another creature as cover to remain hidden.
So once the kobolds manage to hide (somehow) then they can stay hidden by moving into squares filled with bushes.
Here's how I think it should play out: the kobolds hide before the PCs come around the bend (are placed on the map), which means they had total concealment from the PCs, allowing the Stealth checks. Then they should jump into the bushes to be able to remain hidden until the PCs are in position.
Sage's Edit: Stealth is complicated enough that it has its own stickied guide at the official 4e rules Q&A forum.