Wyvern
Explorer
The wording seems pretty clear to me:
"Opening the chest without the proper key causes the needle to spring out"
Lockpicks are not the proper key. So yes, they cause the trap to spring, unless:
I suppose it's not *that* unclear, if you assume that picking the lock counts as opening it without the proper key. (As DEFCON 1 points out, you could make a case that picking a lock allows you to to "trick" the lock into "thinking" that the proper key is being used, but for the sake of argument I'll grant the point.) Even so, it's not as clear as it could be. Look at it this way: first we're told, in effect, that successfully picking the lock triggers the trap. Then, two paragraphs later, we're told that *unsuccessfully* picking the lock triggers the trap. (And none of this was helped by the fact that the adventure in question omits the words "without the proper key" and doesn't say anything about where the key can be found.)
If I'd been editing it, I would have changed it to "Attempting to pick the lock triggers the trap" (assuming that *is* the intended meaning), which is less ambiguous and more concise.
And failing the disarm-check obviously also springs the trap.
Only if failing to disarm a trap automatically sets it off as a general rule; the description of the needle trap doesn't say anything about whether that is the case or not. By contrast, the descriptions of the collapsing ceiling and falling net traps state that "On a failed check, the trap triggers." The fact that the description of the needle trap *doesn't* include this line could be taken to mean that a failed check to disarm the trap *doesn't* trigger it, or the omission could just be an oversight. In any case, it's not "obviously" true.
Failing to pick the lock in the first place would probably also spring the trap, but that is up to the DM to decide.
Of course the DM can decide whatever they like, but going by the RAW, it very clearly and unambiguously states that "Unsuccessfully attempting to pick the lock triggers the trap." No "probably" about it.
Where these sort of traps usually fail, is that they don't take into account 'how' the player goes about opening the chest, or disarming the trap. If the player just uses a crowbar from the side, that needle is not going to do a thing, even if this does trigger the trap. Likewise, if the player decides to keep his hands away from the lock while trying to disarm the trap, and fails, then the needle shouldn't hit him.
I don't view that as a failure; I view it as rewarding the players for using their brains. (Though I'm skeptical whether it's possible to remove the needle from the lock without putting your hands near the lock.)
Having the needle pop out of the lock is awfully specific, and very prone to miss its target.
Again, I consider that a feature, not a bug. In fact, I could argue that making the damage unavoidable if the trap is set off is a bit unfair; in 3rd edition, it would have made an attack roll to see if it hit you. However, that's an issue of design rather than wording. And since you can avoid the damage by wearing gauntlets, using a crowbar, etc., I'm not too upset about it.
As to the question of whether PCs can notice the trap without actively searching for it, I think that would depend on whether the needle springs from the keyhole itself or from another opening. If it's the latter, I would allow someone with sufficiently high passive Investigation to notice that there's an extra hole in the lock.
Wyvern