What it is
Watchdog agent that observes other agents and instruments for drift, scope violation, or quota anomaly. Sister concept to Juniper-as-router and Discipline Engine: where those enforce, Sentinel watches and reports.
Designed to be the "did anything go wrong" surface for the always-on suite.
Where it sits
SENTINEL is catalogued under Infrastructure & Platforms in the instrument registry. Instruments in this group share a concern and are frequently composed rather than used alone; the related entries below are the nearest ones.
This is a registry entry: a real instrument, documented here, without a public endpoint a stranger can open. Pages carrying source excerpts and measured results are marked as such in the catalog. If you want detail on this one specifically, ask about it directly.