If such person or persons cannot be served by any of the aforesaid
methods, after diligent search shall have been made for him or them,
then such notice or order may be served by posting the same in a conspicuous
place upon the premises where such violation is alleged to exist,
or to which such notice may refer, or which may be deemed unsafe or
dangerous, which shall be equivalent to personal service of said notice
upon all parties for whom such search shall have been made; or