Not a theory just a partial observation. I think the coded message is fake through and through.

The reason for my suspicion is the following. Cho, Rigbsy and Van Pelt were searching the wrong room!

When we see Bertram walking to his storage space we see that they are numbered. He is walking to what must be space number 14. We can also see that storage space 12 is opposite space number 20. It follows that 13 is either opposite 19 or 21 and 14 opposite 18 or 22.

When the three are searching we can see that the opposite storage has the number 21. Therefore the 3 are finding the coded message in store 13 not in 14 that Bertram used before.