The description of the limb modification (grapnel arm) of the brawler archetype constructed pugilist, in part, says
On a successful hit, the grappling hook deals no damage, but it functions as though it had the grapple weapon special feature, except it requires only a hit (not a critical hit) and the grapple ends if the constructed pugilist moves more than 40 feet away from the grappled creature. The constructed limb can’t be used to make melee attacks until the grappling hook has been reloaded (a standard action).
(Emphasis mine. Similarly, the weapon property grapple says, "If you move far enough away to be out of the weapon’s reach, you end the grapple with that action"—also see Weapons) Thus a constructed pugilist need only move 45 ft. away from her foe to end the grapple, freeing the grappling hook from the foe (or, generously, a protrusion, wall, or whatever). Alternatively, because the constructed pugilist started the grapple, she "can… release the grapple as a free action, removing the [grappled] condition from both [her] and the target." Either way, afterward, the constructed pugilist can take a standard action to reload the grappling hook so that the limb can be once more used to make melee attacks.
(By the way, after freeing the grappling hook yet before reloading the it, the constructed pugilist seems to be able to still make ranged touch attacks with the grappling hook. After all, ranged touch attacks are not melee attacks. This reader assumes that if this is allowed that the grappling hook and its chain are simply hurled rather than somehow launched as the grappling hook normally is after it's been reloaded.)
On grappling hooks and similar items
This reader agrees that the traditional and not-attached-by-a-chain-to-the-user's-arm grappling hook in its description offers no means of acquiring it again once it's been used, instead saying only
Throwing a grappling hook requires a ranged attack roll, treating the hook as a thrown weapon with a range increment of 10 feet. Objects with ample places to catch the hook have an AC of 5.
However, this GM would rule that it's unnecessary for the game to provide information on how to retrieve a grappling hook for the same reason it's unnecessary to provide information on retrieving any other item that excludes specific mechanics for retrieving it: absent specific rules to the contrary, anything reasonable—like the typical grappling hook—can usually be picked up by the typical creature if the creature takes a move action that provokes attacks of opportunity to pick up an item (see Combat).
(This GM would only use the grappling hook rule that says that It takes 1 minute to pry a grappling hook free (Pirates of the Inner Sea 18) during boarding actions. In the same vein, this GM would ignore the extraordinary ability innovation (grapple launcher) of the gunslinger archetype experimental gunslinger, those rules being unique to that archetype rather than appearing to be applicable generally.)
In my campaign…
With all this in mind, this GM would run things as follows:
- A constructed pugilist can either take a free action or use her speed to move more than 40 ft. from whatever she's secured with the grappling hook of her limb modification (grapnel arm) to free the grappling hook from whatever it is she had secured it to, including, for example, creatures and structures. Thereafter she can take a standard action to reload the grappling hook. (I'd typically have nothing interesting occur between freeing and reloading, but others might—see below.)
- Any creature that succeeds on a ranged attack roll to use a grappling hook for climbing and holds the line to which the grappling hook is attached can take a move action that provokes attacks of opportunity to dislodge the grappling hook and recover it and and the attached line.
Although I suspect it wouldn't often see much use, I'd also allow a constructed pugilist to take the pick up an item action to recover her line and grappling hook the same way she would any other typical object, imagining the constructed pugilist quickly gathering up both the 40 ft. of chain and the grappling hook as a lone attack of opportunity-provoking move action.
In such a case, merely scooping up the grappling hook without reloading the grapnel arm doesn't have any significant game-mechanical benefit: reloading the grapnel arm still takes a standard action. However, if she's out of standard actions, picking up the hook and chain may provide a narrative justification for, for example, why the guards don't take hold of the chain to restrain her when she's running away, why the lava-filled abyss didn't destroy her dangling chain and grappling hook, or why she's not limited in her ability to flee after the portcullis slams behind her.