In my recent game something happened which very much I think speaks to the issue in your OP re DM fiat with the
Alarm spell.
2 NPCs charged with tailing and bringing a PC to a named NPC of Sigil (Estavan) went terribly wrong.
PC killed one of his tailers, the other managed to paralyse the PC and took his backpack informing him if he wanted it back he could find it at Estavan's manor. The backpack had 1 particularly valuable item within.
Some hours later the PC rocked up at the manor.
Estavan had the time, resources (Consortium merchant) and the capabilities (Ogre Mage) to figure out the most valuable item in the backpack. Estavan allowed the backpack to be returned but without the item.
Negotiations were initiated. Estavan wanted the PC to remain in Sigil to find and investigate the other two
doors from the 2e module
Doors to the Unknown. This was an impossible task for the PC as he had to return home...
After some tough negotiations, the PC agreed to answer Estavan's questions on the 2nd door he had found and investigated in exchange for an hour or so researching the item. The PC claimed he wished to
Identify + document his research and thus needed the time.
Now Estavan is an extremely intelligent opponent, at the time of play since this was an unexpected turn of events, I had not come up with the ways the Ogre Mage would have safeguarded the asset should the PC had tried to flee with it, but I expected that Estavan would have been more than prepared. The PC would have surmised the same thing...
At the very minimum the PC knew there was an invisible creature or two in the room wherein they were negotiating.
Estavan having agreed to the PCs demands, left him in the lounge
alone with the object.
The PC's plan was simple - to use the Weapon Bond class feature of the Eldritch Knight. Now the item was not a weapon but a rod (actually, part of a rod

), but I allowed the ability. It was cool and fun idea by the PC.
Sure enough, before leaving Sigil, the PC used the ability to summon the rod to his hand and then he left, plane-hopping for a number of days...
After the session, I did some research about magically "marking items"
I came across
Arcane Mark and
Instant Summons.
- It would make sense that Estavan would have made the necessary preparations for the safeguarding of the rod. He was also the type of person to scry on the PC and use the Instant Summons once he made his way home.
- Now, long story, but we had gone back to flesh out this Sigil + plane-hopping storyline, so the actual campaign timeline is some weeks beyond this point. In that future timeline, the inference of a rod was not present. That is not to say that I would be "taking" this rod because of that, but should I exercise it, it would make sense with the storyline as it progressed.
- The future plan was and is for the entire party to eventually get to Sigil, so this Estavan and the rod would likely be part of a future campaign.
How would you have handled this?
I mean this Estavan meeting and the inference with the rod came out of nowhere. So I had no time to prepare nitty-gritty stuff, besides the main crux of the session was to see how long (and if) the PC made it home back from Sigil to the rest of the party.
[There was a fail-safe which was player-facing, should the character have died, since we were dealing with a past timeline, but that is beyond the scope of the issue we are addressing]