1 - The cable finder is of poor quality. String an extension cord along the wall/floor somewhere, plug it in, and observe how the tester reacts to your attempt to locate an already revealed energized cable. Then stretch another ext cord about 12" (30cm) away side-by-side (in parallel). See how the cable tester reacts to two (2) energized cables approx 12" (30cm) away from each other. Separate the two cable now to span of 24"....how does the tester react?
2 - If in #1 you find the cable tester is trustworthy, then that wall behind your sink must have several cables. Opening the wall to inspect is the only option to ensure their presense.