One morning a couple of weeks ago, I handed my iPhone to my wife and asked her to help with with a privacy experiment. She would use my handset to track my location for the next few days, and with only the software I already had installed. Like a lot of couples, my wife and I know each other's phone PINs. So I left her with the device as I walked into our bathroom to take a shower, simulating an opportunity that I figured would present itself daily to snooping spouses.

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