After reviewing the logs and double checking CM's Marine Law and SOP, I feel the note is valid.

The note in question reads:
Warned for breaking Marine Law as MP. A marine was trespassing during briefing, and marine lost their headset. MP decided to confiscate the headset and tell him he needed to come to brig to get it back instead of returning it when asked by the marine.

In CM's ML, confiscation typically occurs in cases where an item is Contraband (illegal outright) or FFtP (equipment that is legal, but not authorized for the person possessing it).
Collecting evidence for an arrest doesn't apply here as it's stated the MP does not intend to arrest the PFC.

On the comments that the MP intended only to hold the headset while going to the brig to set the PFC for arrest so they could issue an NJP, and to consult with the CMP; please be aware that if you intend to give an NJP you can just inform the person without marking them for arrest. You can use the MP channel of your headset to radio for that warden or another MP to mark someone for arrest for you, or to ask questions within the department, and there are security checkpoints right next to briefing which have both the security consoles and MP/Command locked doors.

I imagine a PFC may be as skeptical of, "yeah, I have your headset, come to the brig and get it" as an MP would be if a PFC was like, "sure thing buddy, you can come collect your gear from me on the DS".

On the matter of theft; As per our wiki the definition of theft is "To take items (or property) from another person or entity without their express permission, or to retain possession of items that have been taken without permission." as it has been established that the MP had no valid reason to remain in possession of the headset, either by intent or misunderstanding of how MP procedure works.

As such, I don't see any reason to overturn the moderator's decision on this note.

Report is denied.

As an aside- I did my best to carefully consider this and be as through as possible. But I recognize that some people may look at this response and have the reaction of, oh, there's management, bending over backwards to find some ML clause to justify the note, so I'm going to try to clarify my reasoning. First, an MP player not knowing or understanding ML does not protect against IC/OCC reprimand. Second, if we decide this is too much in a grey area to be theft, then what we are left with is griefing- as that would be an MP player using their authority to inconvenience a player, outside of ML procedure, rather than properly attempting to arrest them or issue an NJP.