[nandhp]

Probably those apps open pages in the browser (reddit posts certainly qualify), so this could happen if he uses Chrome (or uses Chrome for custom tabs) and has sync turned on. Chrome's browsing history can show up in Web & App Activity if the option "Include Chrome browsing history and activity from websites and apps that use Google services" is turned on: https://support.google.com/websearch/answer/54068?co=GENIE.P...