This is returned when the process you are looking for is not found. It's also used for other things that don't exist in the filesystem not being found too, for instance, if you try and delete a route from the routing table and that route doesn't exist, you'll get a confusing "Process not found" message, see route(8).
lib/main.php:944: Notice: PageInfo: Cannot find action page
lib/main.php:839: Notice: PageInfo: Unknown action
lib/plugin/WlugLicense.php:99: Warning: Invalid argument supplied for foreach()