Thank you o7You mean "Tip Off" missions (those leading to TO-sites)? Very old but still good post by Ian Doncaster - https://forums.frontier.co.uk/threads/investigation-into-the-mysterious-stranger.443594/#post-6945636. You can check the links section in the root spreadsheet of the project for more info on-topic. Donations, courier missions,evacuations and other repetitious activity proved to be a good source for TO missions. Take into account "geographical check" and the fact that missions are "secondary" - they are directing commanders to an already active sites, they do not activate sites themselves.