Sentinels

noun

1. A sentry, watch, or guard.
2. A private soldier.
3. A unique string of characters recognised by a computer program for processing in a special way; a keyword.

   "The tag is a sentinel that suspends web-page processing and displays the subsequent text literally."

4. A sentinel crab.
5. A sign of a health risk (e.g. a disease, an adverse effect).

   "sentinel animals can be used to explore endemic diseases."