Look for any existing client event scripts in your project. There should be something like this somewhere in it:
if system.util.getInactivitySeconds() > 15 and not system.security.isScreenLocked():
system.security.lockScreen()
elif system.util.getInactivitySeconds() > 30:
system.security.logout()
You could also try Find & Replace (Ctrl + F): Find and Replace - Ignition User Manual 8.1 - Ignition Documentation
I would search for getInactivitySeconds, to look for calls to system.util.getInactivitySeconds - Ignition User Manual 8.1 - Ignition Documentation