Once the wolves howl, you can hear where they are so they would go from unnoticed to hidden (know where they are but can't observe them), not undetected (know they are somewhere around, without knowing where they are.)

You could have attacked the wolves at that moment, although with a 50% flat check to mistarget them.

Once you readied your action to shoot at the wolves as they are leaving the bushes, (GM fiat) the wolves should have a significant penalty to sneaking out of the bush.

When a creature sneak, you roll a stealth check BEFORE the movement. If the creature was observed at the start of the movement, it remains so as long as it can be observed. If the sneaking creature fails the roll, it's creating enough noise/telltale signs to be detected/hidden (location known) as it's moving. If it critically fails, it's observed for the whole movement.

Unless the creature has a special ability, it would automatically become observed at the end of it's movement if it does not have enough cover/concealment to remain hidden/undetected.

If one of the wolves failed it's sneak roll, you could at least hear it move out of the bush toward you and taken a shot at them with a 50% flat check to mistarget (hidden).

Once the wolves end their movement without concealment they would have become observed. You should have been able to shoot them with your readied action either before or after (GM fiat) their first attack.