"Canary in coal mine" in a word

Consider tocsin

A warning; an omen. [American Heritage Dictionary]

It also means alarm but the ominous quality makes it appealing in this case.

The canary is a telltale, "something that indicates or reveals information" or "a device that indicates or registers information".

Example in the news: "Tiny wireless sensing device alerts users to telltale vapors remotely". The word can be used as a noun or adjective.

Bellwether, I believe, would be appropriate for such a use.

bell·weth·er
ˈbelˌweT͟Hər

• The leading sheep of a flock, with a bell on its neck.
• An indicator or predictor of something. "college campuses are often the bellwether of change"

synonyms: harbinger, herald, indicator, predictor

From: oxforddictionaries.com

I think just "Canary" can be used here, this usage has become so common when one says canary people will understand that you refer to this meaning. Have a look at usage 6 here http://en.wiktionary.org/wiki/canary

The canary is acting as a guinea pig, in that term's sense, “A living experimental subject” [wiktionary]. The term lab rat has a similar sense. (Also see What's another word for Guinea Pig, i.e. when you call someone a "test dummy"?.)

You might also consider the term point man, in its sense “the soldier who takes point; the soldier who assumes the first and most exposed position in a combat military formation; the lead soldier/unit advancing through hostile or unsecured territory”. This term also appears in wiktionary's example for use of the previously-mentioned word expendable:

Private Johnson was afraid the Lieutenant considered him an expendable, since he was always picked as point man.