Solving the riddle "Hidden Message #3" from The Science of Deduction I've found the solution.
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
2 weeks ago