It turned out that it is encrypted with pigpen cipher.
The pigpen cipher (sometimes referred to as the masonic cipher, Freemason's cipher, or Rosicrucian cipher) is a geometric simple substitution cipher which exchanges letters for symbols which are fragments of a grid. The example key shows one way the letters can be assigned to the grid.
The solution to the riddle, using the picture above is obvious:
Sherlock I have found you
