Well, only way to verify is to dig it. Detect mode has a physical limitation where the machine just cant pick up any deeper. Pinpoint mode, I believe, has the ability to go a bit deeper and thus may produce a "target" where detect mode doesnt register anything. But how and why did you decide to pinpoint that exact spot if detect mode didnt indicate anything was there? Have you gone to other spots and used pinpoint mode and it comes up with a presumed target (which detect mode doesnt indicate)? Or do these other spots not indicate anything, even in pinpoint mode? Could be a glitch in the machine with pinpoint mode always indicating something", no matter where you pinpoint. But if it pinpoints in the spot you first mentioned, but doesnt indicate a target in other locations, then I would say there is probably something actually there in the location you first asked about.
