Over the life of the ALVINNN!!! and The Chipmunks television series, Simon has shown more and more hidden areas to Alvin, usually with some reluctance. The origin of the rooms and compartments has never been mentioned, but they add a unique and mysterious feel to the CGI-animated version of the Seville House while allowing places for the blue-clad chipmunk to store and/or work on his newest inventions.
Sub-Closet Room (Simon's Closet Safe)
A Is For Alien
The first secret area was shown in A Is For Alien and is accessible from just outside the closet in The Chipmunks' room. Following the press of a remote's button, the closet lifts and a hidden closet appears from below before moving away to open up a large white-lit room. This particular closet contains three jetpack suits, one for each brother, and, despite the jetpacks being seen in other episodes, the room has only been opened onscreen once since its debut.
This room, referred to as Simon's Closet Safe by Brittany in The Temp, is briefly seen when Brittany borrows one of the jetpacks to sneak off to school. In this instance, access is through a keypad combination (4-8-5), possibly as increased security, though the interior doesn't match the first screen appearance of the room nor does it appear from below or open the full width of the closet. The room was either confused in some part with the Closet Room or it was rebuilt.
What A Gem
Two side-by-side safes are revealed in What A Gem (only the keypad safe is used in this episode), accessible from the wall cabinet next to Simon's bed (exactly how they descend hasn't yet been shown). After entering the combination (5-5-9-6), the keypad safe, the one to the left, opens. In this episode, Alvin secretly opened the safe to get spy gear so Theodore could infiltrate Miss Croner's house.
Basement Room (Simon's Workshop)
The hidden basement room, styled in the same fashion as the sub-closet room, first appears in Munk Man and is accessible from the basement wall next to the water heater. The wall lifts once a button, the fifth heater knob from the wall near the floor, is pressed. This room is assumed to be the only room Alvin doesn't know how to access, despite his attempts, as Simon refused to show him and he's yet to open it himself onscreen. The room contains the Munkmobile, a flying car, and was built with a sliding roof under the tree outside of The Chipmunks' bedroom.
The room, nicknamed Simon's Workshop, is used again in Super Girls when Simon and Jeanette discuss potential problems with her superhero outfit (as well as seen in a flashback to when Simon was initially testing the Munkmobile).
Alvin also attempts to access the room to show off Simon's inventions to his classmates but is blocked by additional security. Since Munk Man, Simon added a keypad combination, which he's changed at least once (Alvin is convinced after months of trial and error the combination is 9-1-7-2-1-1-5-9, then 9-1-7-2-1-1-9-2). Following multiple failed attempts, a monitor of Simon appears to entertain Alvin with Hawaiian nose humming.
Simon's Workshop makes a reappearance in The Temp. Following a ride in the Munkmobile, simply named Simon's drone in the episode, by Alvin and Simon, Brittany accesses the room to take the vehicle for a secret ride to school. She obtained the combination (4-3-8-2-4-6) from accidentally acquiring a paper with Simon's passwords when he collided with her in the hallway. Simon changes the password again when he believes Alvin damaged the drone on a joy ride.
Closet Room (Simon's Lab)
This third room was first seen in Double Trouble, accessible from the back wall inside The Chipmunks' closet. On close examination, the room is above the sub-closet room seen in A Is For Aliens. This hidden chamber is by far Simon's most secure requiring a ten digit, keypad combination (9-2-6-1-2-4-1-4-2-4) and is equipped with laser beams sensors. In the episode, the room contained a new Simon invention - unpoppable bubbles. Despite the tighter security, Alvin still managed to bypass the first two barriers before being electrocuted by a secret trap around the invention, which, throughout the episode, was believed to be a cloning device.
This room is also seen briefly in Super Girls though the keypad combination is shorter and different from the original code, which would imply Simon changed the code, but Alvin learned the combination again. Alvin opens the room to change into his superhero costume first seen in Super Heroes.
The room appears in Overlooked a couple times and shows more of the interior design. Its security includes a four digit, keypad combination but with the addition of a face scan. Simon uses the room to create a video for introducing, as well as storing, his latest invention - the invisibility suit. Once Alvin learns of the amazing invention, he uses Simon's code and a 2D image of Simon attached to a stick in order to enter and borrow Simon's prototype.
In Wacky Wednesday, Simon uses the room to rebuild his teleportation ray and tests it on a cat, though is body swapped with the cat in the process. The room is once again secured with a short number combination by a keypad.
In Simsky, Simon uses the room primarily to perform surveillance on graffiti hotspots to learn the identity of a graffiti tagger. It's also revealed that he falls asleep in the room frequently, nicknamed Simon's Lab in this episode. Alvin is also seen with the ability to open the door to the room, showing Alvin has learned the door's combination once again.