It’s easiest to play detective on an Android phone. Install the app HiddenEye, and if someone incorrectly enters your passcode, it snaps a picture. This is all done very stealthily so the snooper has no idea he or she is being photographed. The next time you fire up your phone, the photo gallery will have pictures of anyone who’s attempted to crack your password. You can also set it up so the photos are stored remotely via Dropbox. This adds an added layer of security if your phone is stolen or out of your possession.
On an iPhone the options aren’t as good. The best app is called WhoSnooped. It costs $.99 and is more of a novelty than a forensic tool. It doesn’t run all the time; instead you have to set a trap. You open up the app and then lock your phone, leaving it out for the snooper to find. When they slide the lock screen button, the WhoSnooped app opens up and takes a picture of them while displaying the banner “Stop Snooping.” The snooper could easily delete the photo in the app, leaving you with no proof of intrusion. That said, the app let’s the snooper know they are being watched, and it may deter future attempts.