Thanks Tim.
It wasn't doing it every time, just enough for me to notice it. For quite a few years now I've owned cameras that aren't known for their great battery life so I've developed a habit of turning the camera off between shots or groups of shots depending on what I'm doing. I'm probably putting more start/off cycles than normal. Occasionally I'll flip the camera on to take a shot change my mind and shut it off, only to spot something new and flip it right back on again. I thought maybe the camera firmware was not reacting as fast as I expected when doing so. When I started paying attention, that's when I noticed a shutter click before the lens retracted. No image is stored to the card when this happens. I could easily see where there's something in the firmware that is triggered every X-number of on/off cycles, like sensor cleaning, and if I'm walking around turning the camera on and off a lot, that would trigger it more often than expected.
I never notice a random shutter click after turning the camera off when using a manual lens, nor have I heard it with the 55-210 kit lens. It seems to be something that only happens before the lens retracts with the 16-50 kit lens and even then, it only happens occasionally.
I've cleaned the contacts but haven't used that lens recently. I'll try to keep track of how often it happens if it continues to see if I notice a pattern. Both the camera body and lens have the latest firmware, and besides this occasional mystery, both work just fine.