Permanent long-range alarm?

[Jon_Dahl]

There is an NPC drow in my game that will be instantly informed if his hidden treasure vault is breached. Usually the distance between the drow and the vault is somewhere around 700 miles (when the drow is at home). Could someone kindly explain to me how this type of alarm would be possible in the game mechanics?

[zlefin]

I'm not sure what the interaction of trap rules is with casting times. Sending normally has a 10 minute casting time, but does that apply if it's part of a resetting magic trap?

sending connected to alarm as part of a resetting magic trap might work, I just don't know.

[Jon_Dahl]

I'm not sure what the interaction of trap rules is with casting times. Sending normally has a 10 minute casting time, but does that apply if it's part of a resetting magic trap?

sending connected to alarm as part of a resetting magic trap might work, I just don't know.

Thank you for helping me, zlefin. It is interesting to think that how many word 'message' does the alarm 'say'. And the sending spell only works on creatures. What about alarm?
These are tricky questions.

[The Viscount]

If you cast skullwatch you are alerted at any distance on the same plane, and it resets in 1d4 rounds when triggered.

[Jack_Simth]

The Alarm spell has a mental ping option, which works anywhere on the same plane. It can be made permanent with a Permanency spell.

[Jon_Dahl]

The Alarm spell has a mental ping option, which works anywhere on the same plane. It can be made permanent with a Permanency spell.

Mental Alarm

A mental alarm alerts you (and only you) so long as you remain within 1 mile of the warded area.

Sir, asking questions here is not very useful if people don't know the rules but pretend to know them anyway.

[Fizban]

And it's also a part of the spell I expect barely anyone ever runs into, so you probably should have put a reminder in the OP. I certainly wasn't aware of it except by inference.

Watchware from Unapproachable East is probably the spell you're looking for. It lack's Alarm's advantage- Watchware explicitly counts as a magical trap, while Alarm is actually just a spell effect unless the DM deliberately decides its a trap- but is permanent and gives no limit on range or plane for notification. This can be sidestepped by placing the item such that opening the door will disturb it. Assuming your anti-teleport, anti-digging through the wall, and anti-gaseous form defenses work, then a Watchware behind the door should be pretty well unavoidable.