Yes, the elevator, site of many a hilarious Office scene, is fake. The back wall cuts away!
Something else that’s fake: the picture on the security monitor. It’s a photo printout that’s pasted in!
Each character has their own separate makeup bag for touch-ups. Here is John Krasinski’s. (I mean, Jim Halpert’s.)
More makeup bags.
Truck being loaded with equipment for transport to the picnic location.
Can you imagine a convoy of Dunder Mifflin trucks tooling down the highway?