Forerunner first-aid monitor
From Halopedia, the Halo wiki
Numerous monitors followed and inspected the human survivors of the Onyx Conflict in November 2552, notably pricking Halsey in the right thigh with a needle-like protrusion and testing her blood. Linda-058 destroys the monitor before it can leave believing it had attacked Halsey. Halsey suggests that it could detect exhaled ketones and was simply monitoring her because she was hungry. The Spartan-III Lucy-B091 later encountered the Huragok Prone to Drift which utilised a monitor to inspect her. 
The monitors are described to be a matte-gray cylinder no larger than a wine bottle, and are capable of similar anti-gravity propulsion like Sentinels. Kelly-087, when holding a monitor, remarks that it "feels like it's not here. It doesn't weigh anything". They contain a spongy material which can be saturated by substance samples for analysis. It supposedly has numerous illuminated symbols on its underside - Halsey, upon inspection, notes two blinking blue, and one green-white.
List of appearances