sentry

noun5

1. •
   A guard, particularly on duty at the entrance to a military base.

   • Additional sentries are keeping watch for any signs of an ambush. — 2019
2. •
   An animal like a marmot tasked with alerting the pack to danger.
3. •
   Sentry duty; time spent being a sentry.
4. •
   A watchtower.
5. •
   A form of drag to be towed underwater, which on striking bottom is upset and rises to the surface.