Did a quick Google search and found out that the LRO (Lunar Reconnaissance Orbiter) is orbiting the Moon, but it doesn’t continuously monitor the entire lunar surface for impacts. It’s actually pretty difficult to notice a meteor impact in real time, unlike on Earth where a meteor creates a fireball in the sky. Plus, the Moon is enormous compared to the LRO’s field of view.

So apparently, we had the spacecraft taking images of the area - but we only noticed the crater when we compared the images before and after the impact.