In underground coal mines, a canary was kept in a cage. In the presence of poisonous (and explosive) gas, the bird would perish quickly due to it's small size. The dead bird would alert miners to evacuate the shaft.
In the digital world, a "canary" similarly serves as sort of safety indicator.
For example, some websites would include wording that they were not under the affect of a gag order. When they actually received a gag order (which also prevents them from disclosing that they did receive such an order), they would delete said wording from the site, thereby alerting the users that fuckery was afoot.