Yellow lights on vehicles usually mean the same as a yellow traffic light, basically its saying "Hey, pay attention. Caution".

Roadside workers, construction, utility workers, cop cars (depending on where you live), etc have em.

I'm not sure what vehicles you're looking at, but if its the "inspection" photos, its quite possible that Health Canada had the utility companies come out to verify X, Y and Z. The biggest thing about government inspections is ALWAYS about the little nuances and sometimes insanely trivial stuff.