What happened?

[Andy]

Tim is probably right in his assumption regarding the libero and an improper replacement. The penalty is correct, also.

Personally, I would have a real hard time assessing a penalty here based on what I "thought" might have happened without any type of documentation in the scorebook, L tracker sheet, or my lineup card.

I would probably handle it much the way you did. Call both coaches over and explain what happened, you can't confirm anything, no penalty will be assesed, and please make sure that proper procedures are followed for substitutions and libero replacements. I would also have a conversation with my table crew to all stay alert for the libero replacements and make sure they are documented correctly.