Making all employees wear hi-viz, regardless of actual activity is pointless and counter-productive. The mindset created is 'I have to wear this vest for safety. I am wearing my safety vest so I am therefore safe!'. This is possibly worse than no hi-viz at all and actually retards any active and conscious risk assessment of activities conducted during the working day.
The situation I highlighted in my OP may be due to workers having to cross the loading area to get to the canteen, where it has been deemed beneficial for hi-viz to be worn so people can be easily spotted by the operators of heavy moving machinery and vehicles. Issue HV to all workers to be worn all the time and nobody thinks twice about crossing the danger area. Make them don a vest/jacket from a hook by the entrance to the area and suddenly it is rammed home to the wearer that they are entering a potential danger area and they need to pay attention. Which scenario is more effective? (obviously providing a route that avoids the area altogether is the best option but in the real world this is not always possible/practical).
A similar situation exists at my place of work. Some processes are carried out using open baths of acid. People working in that area obviously have to wear appropriate PPE but others who may visit the area for other reasons only need to wear eye protection. To be blunt, the risk of getting splashed in your eyes when not actually processing work is so small that it is almost irrelevant but the very act of making someone stop and put on a piece of PPE as they enter the area makes them think 'This is serious shlt, I had better pay attention, not touch anything and not mess about!'. It works really well.