Our BSA policy states that all 314a requests and any related information will be kept for 5 years in accordance with general BSA guidelines. Somewhere I thought I read that this information was not to be retained in a bank file after research was completed and all that is required is that a bank must be able to shown that it has proper procedures in place regarding the research of these requests. Is there any required retention period for these records or are they not to be retained?

You are not required to keep the actual names included in the 314a requests but you do need evidence that you performed the searches.

What would you consider to be "evidence"?

"Evidence" is in the eye of the beholder. If you issue a memo to a business line to search their records for a 314a suspect and you document the results of that search, I would consider that "evidence". I would imagine that your bank's auditors would rely on the same documentation to determine your bank's compliance with 314a.

What I have seen is banks that keep the reference number and date of the request, the communication regarding the search, and any reports, including "NIL" reports for the names that are associated with that reference number.

Here's a checklist that's on Bankers Tools:
http://www.bankersonline.com/tools/compliance/314checklist.html

We maintain evidence in a log form as well. The log contains each dept with verification responsibilities and documents the date the dept confirmed completion of the review and name of person responding for the dept.
Retail [x] No Match mm/dd/yy John Smith
Trust [x] No Match mm/dd/yy Jane Doe
etc

We also make notes as to any slackers and retraining efforts. Our regulators like the detail and we can easily document that we are meeting required deadlines.