When Johnston talks about the failure of the sentinel in Iran, he sounds frustrated.
They may be taking some action in the future, given the weaknesses that appear to have been exposed in the sentinel.
A sentinel is present 24 hours a day, seven days a week, 365 days a year, including holidays.
A web promotion for X-Men: Days of Future Past indicated that the character had been killed by a sentinel in 2011.
That might have been what happened to the sentinel that landed in Iran.
The sentinel challenged, and Silent went forward and gave the countersign.
It was as natural a part of the landscape as the sentinel cactus.
Might it be a sentinel at the castle who was ordered to go about?
Then suddenly the sentinel on the stairs fell down and we ran out.
Have you read the editorials in the Winchester Courier or the morning sentinel on this case?
1570s, from Middle French sentinelle (16c.), from Italian sentinella "a sentinel." OED says "No convincing etymology of the It. word has been proposed," but perhaps (via a notion of "perceive, watch"), from sentire "to hear," from Latin sentire "feel, perceive by the senses" (see sense (n.)).