On-Access Scanner

When I log on to my iMac, I open the Sophos preferences window and do a manual update. I notice that during the update, the On-Access Scanner will suddenly turn itself off and the padlock at the bottom of the window will automatically unlock. The green button stays red, with the message that the computer is not protected. The lock stays unlocked until I click on it, enter my password and close it manually.

I've also seen this happen when Sophos updates automatically.

Is the On-Access Scanner supposed to turn itself off, and the lock supposed to unlock itself? If so, why?

Thanks!