When I had this problem years ago, I resolved it using my own improvised detection method. And it worked 100% accurately.
Before I discuss the method let me to point out about the limitations of the standard gadgets being sold for this type of detection.
Voltage detector: it may detect the hot/live wire but tell you nothing about the neutral/return wire. Normally both hot and neutral are bundled together but this not necessarily be real for all installations. Using this, you may end up drilling into a neutral wire.
Stud finder: works well in detecting ferromagnetic materials such as iron nails or steel screws. Its sensitivity drops sharply when being used to detect buried copper wire. I might be wrong on this but I don't have much confidence on this gadget to detect buried thin copper wires.
My method; Listen to the line frequency of electromagnetic wave emitted by buried wire when it carries a large current.
I improvised an old "walkman" type cassette player by hooking up a one-inch small 1000 turns copper coil to the wires originally connected to the player's tape head. This copper coil actually from a primary of a small power transformer, with its iron core fully removed. This modified player become a sensitive magnetic field detector. Whatever signal picked by the coil can clearly be heard thru earphones connected to the 'walkman'. This gadget picks electromagnetic field of AC current carrying wires from as far as 18 inch distance.
Detect one conductor at a time. I load the circuit in such a way so that current flows only in one conductor at a time. This is done by connect a significantly heavy load such as a 100 watts or 500 watts halogen lamp to the live wire of that socket and get the neutral return for the lamp at another socket some where else.
This ensures my gadget can listen only to the magnetic field around the buried live wire. When the pickup coil is sweeped on the wall, the intensity of the line frequency signal peaks at its closest position. This is how I locate the position of the cable run.
Repeat this for neutral wire by loading the lamp on the neutral wire and get it live feed from some other far socket.