Simple Q&A D&D 3.5 (by RAW) XXII

[Mongrel]

A 674b
Yes, I can see you're still confused. If the tree is big enough to obscure the Rogue entirely, the Rogue doesn't have line of sight to the enemy and cannot be hidden (and also cannot attack); instead both parties have total concealment from each other. To be hidden, the enemy must have a chance to Spot you. You can't make a Hide check unless someone can make a Spot check in response; that the basic requirement of this opposed check.

I'll skip the rest, because you've got to understand the basics before proceeding.

So you're saying you can't envision someone hiding behind a tree and peeking out from the side to shoot?

Okay, whatever, let's say they're hidden behind a bush or something instead. Is that better? The actual object they're hiding behind is immaterial, I don't know why you're so focused on it. If it helps, I'll say that the rogue is already successfully hiding behind some appropriate cover. So here are my questions and my example again, ammended for your benefit:

Q674b: So say there's a rogue hidden behind appropriate cover (he's already hidden (i.e. he has made a hide check versus the enemy spot check and hasn't been discovered), he's not trying to hide in the middle of combat. The cover is significant enough to obscure him enough to hide). That rogue uses his light crossbow to shoot at an unaware enemy 30ft. away. If I understand you correctly this is what happens:

Rogue gets attack against enemy at +2 to hit, and enemy is denied dex bonus to AC (which allows sneak attack).
After the attack is resolved, enemy gets a reactive spot check against a DC 3 (0 + 1 for every 10ft away = 0 + 3 = 3). If he makes it he sees the rogue, otherwise the rogue remains hidden.
Next round if he is not still hidden the rogue can use his move action to duck behind full cover and attempt another hide check which, if successful, would allow him to attack with the bonuses of attacking an unaware opponent. Your interpretation seems to indicate that he would also be unable to use the "sniping" action since he doesn't have his move action anymore, but in looking more closely at the "sniping" action I don't see anything that suggests you need to use a move action; it seems like the check is simply made as part of the attack.
Alternatively, the rogue could have elected to try to snipe as detailed under hide in the PhB. In this case, the enemy would still roll against that 3 DC, but if the rogue's hide -20 check beats the enemy's spot check (the same one against the DC 3? A new roll?) then he remains hidden, but the enemy knows what square he was in when he made the attack.

A couple questions:
Q674b: doesn't having cover give you a bonus to your hide check, so the DC should be a little over 3? I looked it up in the PhB but it's very vague. The most concrete thing it says is that near total darkness gives a 40% miss chance and +10 to hide checks. Is there anything more substantial than that with regard to bonuses to hide checks from concealment?
Q674c: The wording of sniping seems to suggest that the hide check at -20 replaces the DC of 3 in the example. Why is this not the case?

Better?