Or CBP could ask "Have you deleted anything from the phone in the last week?" There are a bunch of ways to ask it, and it can put a deleter in a real bind, and raise the 'ole blood pressure if you have to choose between lying or admitting what you've deleted.
I would suggest, as I always do, that you just don't answer any questions in secondary inspection if you're American, including questions about the contents of your electronic devices.