Does anyone know what this is?

It's a refractometer.

It's used for checking the amount of a liquid in another liquid, alcohol in wine or soluble oil in water for example. In this case it's for measuring antifreeze in water.

I checked the coolant in the engine today and we're good for temperatures down to minus 35°C.

The clear plastic window is hinged and lifts to allow a sample to be placed on the blue prism. The clear plastic window is closed onto the sample expelling as much air as possible. Holding up to the light a reading is taken through the scope. A graticule scale of various similar liquid types shows the mixture richness at the blue/white interface.