I like the idea of using the magnet to verify your location below .... there's nothing like opening the wall just one stud off!
Otherwise ... check your assumptions. I had a similar situation not long ago, where both cables turned out to be switch legs, with the feed coming in from the crawl space.
I know this sounds basic ... but nearly every time I've had a 'bad string of receptacles,' the problem has been at the last 'good' receptacle.
The few times the problem was elsewhere, the problem traced to an unconnected wire in a junction box / device box that got buried by the rockers. In this case, you can still find the box with a toner, as the tone will sound through the opening in the mud ring.
Another aid in finding a buried box is a 4 ft level ... to look for small bumps in the wall.
