Can a character with Spring Attack who begins her turn hidden move up to a foe, attack him, then return to a position of hiding? Is she considered to be hiding (that is, invisible to the foe) when she makes the attack? What if the character has the camouflage or hide in plain sight class features?
Normally, a character can’t make a Hide check right after attacking a foe, since that foe’s attention is now focused on her (even if the attacker started her turn hidden or invisible). The sniping option (on page 76 in the Player’s Handbook) allows a character to make a move action to hide immediately after making a ranged attack against a foe at least 10 feet away, but this doesn’t apply to melee attacks (even those made with reach). Even if the character has Spring Attack, she simply can’t make a Hide check while she is being observed.
As far as your second question goes, unless the character’s approach remains entirely in an area where she can hide (that is, an area with sufficient cover or concealment to attempt a Hide check), the character is not considered to be hidden still when she makes the attack. Conceivably, your character might begin her turn hidden in overgrowth, move up through the undergrowth to attack a target, then move back to a hiding place within the plants, having never left the area of concealment. In this case, she’d be considered hidden when she made the attack, although she’d have a –20 penalty on her Hide check.
The third part of your question changes the situation entirely. Separately, both the camouflage and hide in plain sight class features make this tactic more useful, but together, they’re incredibly effective.
A character with the hide in plain sight class feature (described on page 48 in the Player’s Handbook) can make a Hide check even if she’s being observed. This doesn’t require any extra action to accomplish (unlike the sniping maneuver). The character could attack a foe, then move to a place with sufficient cover or concealment to allow a Hide check, making the Hide check as part of movement. The character doesn’t need Spring Attack to pull this off, although that feat would allow her to move (potentially from a place of hiding, although that’s not necessary), make an attack, and then move again to a place of hiding. Still, unless the character has cover or concealment for her approach, she’s not considered to be hidden when she delivers the attack.
The camouflage class feature (also on page 48 in the Player’s Handbook) allows the character to make a Hide check in any sort of natural terrain, even if it doesn’t provide cover or concealment. This means that the character could begin hidden, move up to a target across open terrain, and make an attack while still being considered hidden (although she’d still suffer the –20 penalty on her Hide check). Even if the character has Spring Attack and moves away after the attack, she can’t make a Hide check to hide after the attack.
Put all three of these elements together—such as in the hands of a high-level sneaky ranger—and here’s what you get:
1. The character begins his turn hidden (as long as he’s in natural terrain, he doesn’t even need cover or concealment).
2. He moves up to a foe across natural terrain and makes an attack (making a Hide check with a –20 penalty to be considered hiding when he attacks).
3. He then moves back from the foe and makes a new Hide check to disappear from view (again, he doesn’t need cover or concealment while in natural terrain).
4. The foe then, if still standing, says, “Hey, what hit me?!”
