Using an innocuous encoding of a word is a form of encryption. People who expect to be under surveillance agree on a set of code words to denote illegal things. Though hard, there are multiple ways to semi-automatically break such a linguistic encryption.
Imputation. [1] Remove a word from a sentence then try to predict it from its surrounding context. "when I get home tonight, i vape a ___ then space out". Assign predicted probabilities to imputed word ":leaf emoji:" ["marijuana cigarette", "electronic cigarette", "cigar"].
Active learning. Seed the algorithm with expert knowledge from law enforcement, drug users, and social workers, who know of the encryption keys.
Anomaly detection. Though perhaps easily-confused with other, innocuous usage, street slang is a distinct form of language with its own properties and patterns. Compared to common discourse, it is strange and random. This pattern could be measured.
Doing this rigorously, like building search engines for illegal drugs or human trafficking on the deep web, requires a lot of expert knowledge. [2] Maybe future deep learning can do this end-to-end on arbitrary domains? [3] Let's see.
[1] https://arxiv.org/abs/1312.3005 "One Billion Word Benchmark for Measuring Progress in Statistical Language Modeling"
[2] http://www.darpa.mil/program/memex
[3] https://universe.openai.com/envs#world_of_bits