I recently switched from Banking to the Credit Union world. One thing I noticed is they keep their Original Promissory Notes in the loan files and everywhere i've ever worked we've kept them locked or in a separate location. I really don't have any "back-up" to support why this should be done though. Any suggestions/reasons why we really should?

Your loan files are not locked up?

Promissory notes, which is your evidence of debt, are usually (at the very least) kept in some sort of fire resistent storage device (cabinet, vault, etc.). Without the promissory note, collection of the debt might become sort of problematic. However, this all fails under appropriate physical security and I doubt you will find anything that specically addresses this issue.

The files themselves are locked up but every bank i've ever worked at has always maintained our original notes in a separate area in fireproof safe, etc.

Imaging has changes some of this. As an auditor, I have never seen a bank keep the original promissory note in the loan file.

Even back in the dark ages when we hand typed notes, the original was put in a note jacket and filed in a cabinet in the vault and there was a carbonized copy for the file and another copy for the borrower.