You and your player may be overlooking some things about how Stealth and hiding work. You should both read The Rules of Hidden Club, which explains both comprehensively. It's an excellent guide, and the official rulebooks don't explain it nearly as well.
Bear in mind the Stealth rules in the first Player's Handbook are broken and incomplete. They're repaired and amended in the Player's Handbook 2 on page 222. You can read the updated rules in the Player's Handbook update PDF from the D&D 4e Updates and Errata page.
If your Rogue player wants to hide, merely standing far away is not good enough. They have to meet several other conditions, too. Quoting Hidden Club:
How do you become Hidden? You roll Stealth at the end of any Move Action or any of your Actions (even Free Actions) during which you move.
"Move" is defined in the PHB3 glossary as, roughly, to leave one square for any reason to enter another square. You can't roll Stealth after Forced Movement since that's not your action. You CAN roll Stealth after a granted Free Action.
You must also meet three requirements:
- You must not be visible, which means Total Concealment or Superior Cover, at the end of the move.
- Your Stealth roll must beat the Passive Perception of anyone you want to be Hidden from
- You cannot have HAD Hidden at the start of the action and lost it during the action. Which is to say, you can't BECOME Hidden as part of the same action that loses your Hidden state.
By the way, any time you become Hidden, record the number you rolled. It's important.
In addition, your Rogue player may incur a penalty to their Stealth check for the move action they're using to become hidden. Quoting PHB2 p222's Opposed Check entry in the Stealth skill:
If you move more than 2 squares during the move action, you take a –5 penalty to the Stealth check. If you run, the penalty is –10.
Once you're Hidden, it's not as simple as just staying that way. All your Rogue's enemies still know where they were before he became hidden, and he still has to follow the Rules of Hidden Club (as described in that page I linked) in order to stay hidden. For example, he has to maintain at least cover or concealment from any enemy, or he is no longer hidden from that enemy (who can then immediately inform the others).
So: get reading!
To answer your question directly, yes. You are correct.
From what I understand though, you are confused about a) the "DC" of your hide check, and b) what happens after you are spotted, and targeted.
Well firstly, a creature's Passive Perception is calculated by adding 10 to their Wisdom Modifier. This means that a creature would need a Wis Score of 16 to spot you with passive perception. This is unlikely. However, the GM may have decided to give them advantage, since they got the drop on you. This means that they get an additional +5 to their Passive Perception score, which means they only requires a Wisdom of 8 in total to still spot you. (10 + 5 - 2 = 13)
Secondly, if they do not spot you, and need to search for you, even if only one succeeds, the gig is up. You've been spotted, and your position has been given away by the monster hacking at you. Even a monster with only 8 Wisdom would still be able to figure out that there's something worth attacking, if one of his friends was having a go at something they couldn't see.
Best Answer
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:
However!
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.