Ink, Stink

Source Advanced Race Guide

Stink ink is a special, pungent, musk-based ink that allows its user to encode information with smell rather than visually. Stink ink dries clear but its sharp, extremely localized smell can be picked out by those with sensitive enough noses to make it possible to read by sense of smell. Only creatures with the keen senses trait or Scent ability can read stink ink without aid of some form of magic. Reading or writing something with stink ink takes twice as long as going through the same amount of information written in normal ink.