Thousands of pictures including "naked selfies" have been extracted from factory-wiped phones http://www.bbc.co.uk/news/technology-28264446 Some older smartphones only erase the indexing of the data and not the data itself, which means pictures, emails and text messages can be recovered relatively easily by using standard forensic tools that anyone can buy and download. As with computers... Delete does not always mean Delete
It was recommended by Google that all users enable encryption on their smartphones before applying a factory reset to ensure files cannot be accessed. This feature, said Google, has been available for three years, although it is not enabled by default, which could leave less tech-savvy users open to attack. Apple has had built-in encryption for its hardware and firmware since the release of the iPhone 3GS. The hardware encryption is permanently enabled and users cannot turn it off.