|The garage door is an illusion. The door actually leads out to a sheer cliffside. Once down the stairs, an illusionary fire starts.
Once down into the warehouse, a DC 10 Wisdom (Perception) check reveals that there are some barrels of gunpowder stored here.
When any of the contents of the warehouse are disturbed, a fire breaks out in the corner and begins to rapidly spread towards the gunpowder in 2 rounds. When it reaches the gunpowder, small specks of gunpowder near the barrels ignite and set of mini-explosions. One round later, the barrels ignite and explode, and then the illusion disappears, and will reset in 10 minutes. No damage is done.
If the fire is examined, a successful DC 17 Intelligence (Investigation) check reveals the fire as an illusion. The same check is also necessary to determine that the courtyard outside is an illusion. This examination requires an action to perform.
Any creature that runs out the door must make a DC 17 Dexterity saving throw or fall off the cliff, taking 10d6 falling damage. Those that see the creature fall may make a DC 17 Intelligence (Investigation) check to penetrate the illusion. Otherwise, it just as if the person vanished.
A spell such as Detect Magic detects an aura of illusion on the garage door.
A spell such as Dispel Magic permanently disables the illusion with a successful DC 17 spellcaster check. Any effect that suppresses magic at spell level 7 or above suppresses the illusion on the garage door and prevents the fire illusion for the duration of the effect. If the suppression happens while the fire illusion is active, it disappears.