I felt as thou I should share this information. But that Relic Eye thing brad had in the quick look on top of spotting gems will also beep if you are close to a secret door which leads to the black market (and possibly other places I guess).
You can buy a lot of different stuff there, I even bought a new character (who ended up shooting me with a freeze gun he stole :(
But if you ever see that thing start beeping that is why. Oh and It beeps faster the closer you get