I am sure you looked for her to come back to get the paint. If she wanted to leave the room again after coming back, she should have looked for you. She didn't announce she was leaving the room, so you didn't either. I would let it go unless she makes a big deal about it again the next day or this week. If so, just calmly explain that you never announced leaving rooms before, but maybe you should start to make sure there is coverage for the kids. I know how it feels to let one thing ruin your day and you think about it all day and the next day. The feeling you are feeling will dwindle down in time and you won't feel as bad. Especially if she doesn't make it into a big thing again.