It sounds like there is someone who is confused.
has a "restricted record" that is used to maintain information not contained in your regular service record book. But that is very limited in what it can contain.
An Art 15 would not go into the restricted record.
It would remain in the regular service record book (SRB)
And it is available to anyone who can see the SRB.
Now...under the Privacy Act of 1974, your SRB is protected. No one can view the record without permission or a warrant.
However, most employers will require you to sign a waiver giving permission to access the records.
I would bet that commander was confused and that this is in your SRB. But you can check it yourself. Just request a copy of your record. YOu can do that herehttp://www.archives.gov/veterans/military-service-records/
The National Archives will send it along to you for free. Typically it is 4-6 weeks to process the request.
If they are asking you about it, you can bet they will see it if it is in there...so you may want to disclose it.
Or request it to see yourself.