Anything is possible. Code says that you can't just leave live wires in walls. They have to end in a junction box and the box has to be accessible. Of course, if your house is like my house, there are probably a few things not quite to code. The problem is that the typical detectors only detect a live wire within an inch or so, both because it is harder to detect from farther away and to avoid excess false positives when working around multiple wires. Plus it is harder to detect a live wire behind a metal plate (as wires should be in certain places for physical protection). So:
- Check any blank cover plates. Those are often for a junction box that is used to connect two parts of a circuit, but they can also be places that used to have a switch or receptacle or hard-wired device. Open them up and see if you find live wires that turn off when you turn off the breaker.
- Check inside cabinets. A box is considered accessible behind closed doors, as long as no tools are required to get to it. If a cabinet was placed over a junction box when a kitchen or bathroom was renovated, you may find it lurking inside the cabinet, with a blank plate or a receptacle.
- Check outside. Outside receptacles often sit unused and forgotten.
- Carefully remove the front cover of the breaker panel. (You'll need to that anyway when it comes time to actually add a new circuit.) You may find that the breaker is simply a placeholder with no wires attached. Or you may find a label attached to the wires, or (less likely) be able to figure it out from the direction the wires leave the panel.
If all else fails, you could detach the wires from the breaker, cap them with wire nuts, and label them "unknown from breaker 17". Then use the location (with new breaker if a different size is needed) and deal with the problem of a now-dead circuit if/when you ever figure it out.