I believe this is already fairly well-defined -- "Questions about airport operations" is intentionally broad, but at the same time it should not be construed so broadly as to mean "Questions about anything I saw at, on, or near an airport" -- It's mainly intended to cover things like:
- Questions about ARFF procedures
- Questions about runway/taxiway maintenance
- Questions about airport tugs & such used for moving aircraft
It may also include questions about duty-specific clothing (e.g. the silver fire suits worn by ARFF personnel), but those are at best peripheral to the category of "Airport operations", and if they're closed as off-topic it shouldn't break anyone's heart.
In the particular case of the instant question there are plenty of potentially-on-topic questions about "Why is this person wearing a face shield?" -- mechanics working in a wheel well may be wearing them so falling dirt/hydraulic oil doesn't get in their eyes for instance.
The question we're discussing here doesn't pass muster in my opinion though: We could well be asking why the Brinks driver is wearing a tactical helmet, so I would say this is a case of "Just because it's AT an airport doesn't mean it's RELATED to the airport." and properly closed as off-topic.