sign-in notifications

So I get these occasional notifications on my Mac that "computerxyz has signed in" from Teamviewer. Duh? Why did these computers sign out? I seem to get these pretty regularly. In fact, I sometimes get these messages right after I've looked at the "Show Teamviewer" window, and that machine is signed in.

Yes, I know how to tell Teamviewer not to send me these notifications. What I want to know is why these machines have signed out. Some machines are on my local network, and there has been no power failure or router issue.