Do you still call the room to attention/at ease if you are the only person in the room?

I wouldn’t, nor would I expect it. I would and have come to attention and greeted the more senior person.

You're likely messing with me.....I see this question again and again. So, here is my answer again. Do it - until your superior says not to - or sets the tone. Always error on the side of caution.

Yes you do it as if its a packed house and standing room only and even if its only you.