A free app for Android, by Viticly.
Who Touched My Phone? - WTMP is a mobile phone security app that will help you detect if someone is spying on you. The app can secretly take a photograph when someone tries to unlock your phone. If the app detects anything suspicious, it will alert you and help you protect your device from intruders.
The app takes a picture of the intruder using the front camera of your device. However, the intruder will not know about this feature since the app will be running in the background.