Pay attention to what's written at the top and bottom of the note.

At the top: "How many of your kind will be joining us here soon, Calvin?"

At the bottom: "Ps-Check your mailbox"

In the upper right corner, someone else wrote a note that says: "Note to self: came in envelope w/ animal figurine in brown/orange pouch". What is this referring to? Read the Crime Scene Report and Evidence Logs carefully.

Calvin had a small zebra figurine in an orange/brown pouch in his pocket when he died.

Read the Animal Facts Sheet carefully.

This is a simple Caesar cipher.

This is a Caesar cipher with a shift of 3.

The clue is "How many of me are there?" The note came with a zebra figurine. There are 3 zebras at Riverside zoo (see the Animal Fact Sheet).

Decrypted, the note reads:

Calvin —

Can we keep the gate thing between us? I know I messed up, but it was a one-time mistake, and I really can’t afford to lose this job.

I didn’t want to say this, but if you go to management, I’ll have to say it was you. It’ll be your word against mine — and let’s be honest, you don’t have a lot of credibility anymore.

The constant complaints haven’t exactly won you any friends. Your call.

—Jordan