The iPhones generally store more data than other high-end phones -- and investigators (plus supicious spouses) can tap in to that information for evidence.
And while some phone users routinely delete information from their devices, that step is seldom as final as it seems. When you hit the delete button, it's never really deleted.
Every time an iPhone user closes out of the built-in mapping application, the phone snaps a screenshot and stores it.
iPhone photos are embedded with GEO tags and identifying information, meaning that photos posted online might not only include GPS coordinates of where the picture was taken, but also the serial number of the phone that took it.
Even more information is stored by the applications themselves, including the user's browser history. Clearing out user histories isn't enough to clean the device of that data. Just as users can take and store a picture of their iPhone's screen, the phone itself automatically shoots and stores hundreds of such images as people close out one application to use another.
The keyboard cache logs everything that you type in to learn autocorrect so that it can correct a user's typing mistakes. Apple doesn't store that cache very securely, so someone with know-how could recover months of typing in the order in which it was typed, even if the e-mail or text it was part of has long since been deleted.
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