I know what it sounds like.
No to be argumentative, but there is only two sounds a NON fired slide makes. One is if you slide it all the way back to the locked position, which would be ONE click, and would eject a round, followed by the LOUD sound of it being released, which didn't happen. The other is if it was, as you have speculated, iirc, sliding it back to check if a round is loaded, which would be only ONE loud sound as it was released.
Interestingly enough, BOTH of these options require BOTH hands to accomplish, and that would be pretty tough if GZ was on the phone at the same time.