Thiefsign

Thiefsign is a carefully-guarded artificial language used by professional thieves across the Sundered Lands. It is composed of subtle gestures and postures whose meanings are changed by the gestures, and paired with secret glyphs that may be casually sketched on buildings to carry warnings or indicate rich pickings within. Should a thief attempt to pass on the meaning of any part of thiefsign to an outsider, it is an automatic mandatory death sentence for both the thief and the one whom they taught.
The gestures allow thieves to carry out entire conversations with nothing more than casual movements indistinguishable to all but the closest observer; warning of traps or ambushes, that things are not as they seem, that an individual is an easy mark, claiming a territory, and more. Some law enforcers have been able to puzzle out portions of some of the gestures, but all thieves' guilds are required by their underground law to murder any who may learn or discover this particular secret.
Thieves' Guilds across the Sundered Lands share the language in common, carefully taught by one guild envoy to another, but regional 'accents' have slipped into the gestures of the various guilds. This serves as a useful shorthand method of identification and security; while sufficiently-senior members of different guilds will be able to understand one another, a skilled and knowledgeable reader of thiefsign would be able to identify if an outsider claimed to be from Bajapur when he or she actually had the accent of a thief from the Wyrmsea Republic.
The secrets of thiefsign are restricted, and only taught as a guildmember climbs the ranks within the guild. A bruiser or pickpocket may only know a vocabulary a few hundred common words, while an influential guild lieutenant may know several thousand specialised terms. This also helps to serve as security, limiting the amount that a less-trusted and potentially treacherous lower-level guildmember could reveal.