My folks share one computer for syncing their iPhones and an iPad. iTunes on this computer is not signed in and "sync when new apps are added" are disabled for every device. However, when my folks update apps on their devices, they would sometimes be asked for the other person's iTunes password. I have tried deleting the app from the affected device and redownloading it to hopefully give it the right ownership. I haven't been tracking if it works, as I am no sure if this is just affecting the apps I haven't redownloaded. For now, I'm curious if anyone has the same issue and has found the cause and solution for it.