They probably just monitored what files the spotify client had open, or on Windows uses something like Process Monitor and monitored the program's activities. I

They probably just monitored what files the spotify client had open, or on Windows uses something like Process Monitor and monitored the program's activities. It would have seen a file open command pretty soon. And once they saw it open the sqlite.db, all they needed to do was run sqlite3 on the command line against the DB file and see if there was history in the file. It is possible that the Android client does the same as sqlite3 is the underlying database format for apps and is very easy to use.