Fanaelialae
Legend
It would be exploration if you found that room yourself and decided not to investigate further. You did find out something new via exploration.Exploration to me isn't an encounter. It's finding out about something new by exploration. If you go into a room you've never been in, you're engaged in exploration. If someone says to you, "Do you want to see my room?" and you respond with "no," it's not exploration despite the potential to be. You never explored it. You have to actually engage in the exploration for it to be exploration.
When you travel down the road and the DM describes what you see, you're finding out something new by exploration (assuming you haven't been down this road before, or that nothing has changed if you have).