You can perform a small ritual to write some symbols on a flat surface. You cannot use writing, only symbols, glyphs, small drawings. The markings can convey one simple meaning, akin to what could be done with thieve's can't or hobo signs.

When doing the ritual you can specify any number of persons within line of sight. Any wizard or person designated during the ritual can clearly see the markings and understand their meaning. Others must roll an Intelligence (Investigation) to detect the markings, but cannot decipher their meanings.