Facility X
The quests A Switch in Altitude, Out of the Box, Best in the West and Foul Play were formerly exclusive to a single platform (Nintendo Switch, XBox One, PC and PS4/PS5 respectively), although they are now available on all platforms. They were connected through a multi-platform easter egg, that begins by finding the 
Encrypted Key and a clue at the end of each quest. The clues refer to locations where the key must be brought. All four clues are needed to find the easter egg, and originally needed to be shared between the players on each platform.
Clues
A Switch in Altitude clue
One of the hidden places can be found in Autumn's Fall, "North of a big ruin, surrounded by water"!
This clue refers to the Great Lake in Autumn's Fall, on the north side of the ruins where you fought the Hologram Frobbit.
Out of the Box clue
One of the hidden places can be found in Bergen Trail, "Between a laser bridge and the hidden lair of Frobbits out of reach."!
This clue refers to the Off Road 3 in Bergen Trail, where you will need to take the high ground in the north to get to the small protected glen on the west. Go counter-clockwise around the glen with the two Frobbits in it on the first level above ground. The key is on the 3:00 side of the circle, just outside the wall formed by the second level above ground.
Best in the West clue
One of the hidden places can be found in Maroon Valley, "At a hidden source of water, uncovered while searching for cursed treasure."!
This clue refers to the Deadly Oasis in Maroon Valley, at the top of the small pond area.
Foul Play clue
One of the hidden places can be found in Gaia's Garden, "At a crossing, on top of one of the four tree stumps"!
This clue refers to the Splitting Stumps in Gaia's Garden, on top of the northern-most stump.
Facility X
After you have found all four parts of the decryption key, you will be informed that the secret Facility X is somewhere in Sapphire Ridge. You will find an elevator similar to the first one in Carved Pathway, in the northwestern area; blue lights coming from you will lead the way.
This facility houses the four different creatures found in each respective quest. Meanwhile, Drey Santres, Xavier Bonegate, Dave Halman, and Barney Sauceston can be found in the furthest room playing video games.