It is coming from the canary birds in the coal mines or in submarines[0]. They have a higher sensibility to CO than humans. This is now part of the common language to say that you sacrifice an animal or "something" to get early warning of something possibly more dangerous.

[0]: https://en.wikipedia.org/wiki/Sentinel_species#Historical_ex...

Apologies, you're right, it's not really covered as it's a bit of a niche industry term. It relates back to the days of coal mines, and the birds being used as an early warning system - https://en.m.wiktionary.org/wiki/canary_in_a_coal_mine

Canaries were used in mines to signal pockets of unbreathable gas; they would die before the humans giving them an opportunity to escape.

Canary has developed into a standard term for warnings which are detecting the danger allowing mitigation as opposed to predicting the danger which would allow avoidance.

It's a reference to the canaries that were used in coal mines to detect carbon monoxide. Since then 'canary' has been used to refer to early warning systems.

https://en.wikipedia.org/wiki/Sentinel_species

See here: https://en.m.wikipedia.org/wiki/Canary_trap