JiffyPopTart
Bree-Yark
It looks like GM fiat, which is why I don’t do it.
The player may not "see" anything. But behind the scenes and out of the players eyes the GM in fairness needs to construct a series of events that would lead up to the tuning fork being stolen.You said a lot but didn’t really answer my question. Surely tuning forks can legitimately be stolen. How does a legitimate case of that look to the character as opposed to an illegitimate railroading case?
Without knowing the OPs reasoning behind the "How was it stolen" question all I can do is offer some scenarios.
If it was a physical theft....the GM needs to establish questions like how did the thief know they had one? How did they overcome any special defences the play may have taken (magical chests or alert runes or whatever). Once the narrative of a theoretical plan is created then the GM needs to follow the rules of the game to enact them. This can be hidden die rolls of need be, but the roles need to be made and the players defenses need to be addressed.
Conversely if it was just "When you teleported in your tuning forks were all stolen magically" then the GM is using fiat instead of game mechanics. GM fiat is a tool in the box, but it's also a part of the railroad.
The character very likely wouldn't "see" anything of the fork was successfully stolen how were the player needs to be infirmed if the mechanics of the theft so that they can alert the GM to any special defenses or notes that might affect the success.
I have zero issue with a rogue making a stealth check to beat the characters passive perception scores and then making a successful sleight of hand to swipe the item....but the thief needs to have a narrative reason to know it's there.
I dont have an issue with GM fiat "Well, it's just missing" because of crazy world breaking magic" either. The issue for me is that the latter method is railroading in a traditional sandbox setting.
The extrapolation reasoning....let's say the crazy world spell method was used. What happens when the PCs defeat the creator of that spell and then being them back to the Prime Material and have them recast it but instead just say "Every time someone teleports to the plane I magically steal their precious gems". Sandboxes need to be logically consistent.

