Ah the great door etiquette debate, which I assume relates to work.
It doesn’t matter whether there is one person sitting in their office with the door open or closed, or there are several people in a meeting or small office with the door open or closed; I Knock, even if I proceed to go straight in.
The only time I don’t knock is when entering some kind of open plan office.
Also seniority has nothing to do with it, it is polite. More importantly, it sets you up for a grand entrance so people notice you, and puts them on the back foot since you have undertaken the first 'polite' act before they even have time to acknowledge you.