if ariel keeped her grotto a secret, than who did her father come in fast and distroy her treager?

Sebastian found her grotto in "Part of Your World" so it wasn't a secret to him. Later after "Under the Sea" when Ariel disappears, Sebastian is called to see the King who asks if Ariel is in love. Sebastian admits she went to the surface and Triton gets very mad so he wants to know where she is and Sebastian tells him about Ariel's grotto.