Definition of sentinels:
(n) :
A sentry, watch, or guard.
(v) :
(transitive) To watch over as a guard.
(v) :
(transitive) To post a guard for.
(n) :
(computer science) A unique value recognised by a computer program for processing in a special way, or marking the end of a set of data.
(n) :
(attributive, medicine, epidemiology) A sign of a health risk (e.g. a disease, an adverse effect).
(n) :
A sentinel crab.