In some big hospitals, you can also find staff out of uniform grabbing a quick bite to eat. They probably do it in uniform as well.
Seen quite a few staff grabbing a bite in places you wouldn't expect them to see. They need food and a few minutes away from being asked questions from colleagues and patients. Sometimes there isn't time to go from the 17th floor down to the canteen and back up to the 15th. UCHL, I have spent a lot of time wandering around over the years as an inpatient. Staff sitting on the walkway. Staff grabbing something from the vending machine on the ground floor because their lunch is a couple of floors away. There's a bit of a lull in a&e. and very space to sit.
Might irritate you, or you think it's rude, or whatever. But people have busy lives. You say you would never do it now, but when you've gone hours with no food, and that's a chance to get some food. Don't begrudge them.
Ok, it's 2 pm on a Thursday afternoon. They should have had breakfast, but what if they could"t? When I attended clinics as an inpatient, unless I was in a wheelchair, you wouldn't know other than looking ill, that I was ill.
Sometimes that's when I could eat. Same with other patients, who also wore clothes. Not to irritate others. But because the NIL has finally been lifted that was set just before breakfast. Or been in surgery and now got that hunger after the metallic taste goes, you want food.