What Jack is doing is by RAW different: he shoots, pretty content at being seen. Then, he relocates behind cover. As long as he needs less than half his movement speed to reach cover, he gets no hide penalty. Between half move speed and full move speed, he gets a -5 penalty. Result: As long as Jack succeeds on his hide rolls, opponents have some idea where he just relocated to (they have no line of sight). They could try to aim at him with splash weapons or AoE spells (these require no line of sight). They just can't directly shoot him, as long as he's behind solid cover (no line of effect nor line of sight). They could try to shoot him with total concealment if he hid behind darkness or fog, as long as they have a way to guess his location (line of effect but no line of sight).
EDIT: Or yes, they could try to circle around his cover, until they have line of sight. Or close in to his cover and engage in melee.
That's why grenades are useful against a mobile enemy in close range in urban warfare, and utterly useless against an undetected sniper, whatever the distance.
These are the times I really love the 3.5 rules, as they sometime modelize stuff pretty well.
EDIT: got the penalty for moving between half speed and full speed wrong, it's -5. See here.