Users Failed Firewall Authentication

In daily reports I find Firewall Authentication Notices for users. I noticed it happening when my own user account was on the report during an hour when I was not working (asleep even!). I realized that the majority of my own logs for this occurred on a PC which I had locked, but not logged off. I found logs for my daily PC which I lock, but it goes to sleep after some time. 

After some more digging, I found that some authentication failures are happening while I'm logged on to the PC and using it, but not making any active attempts through the firewall (I'm sure some background objects are though?).  This doesn't appear to be causing any problems, just a build up of logs.