Past perfect vs past simple

Wanting to avoid the roadworks, he decided to take the long way. What he didn't notice/hadn't noticed was that there were roadworks there as well.

I saw this on my exam and wrote "hadn't noticed", but my teacher said it was incorrect. How so? As far as I can tell, the noticing happened before he decided to take the long way, not after.

P.S
This is not exactly what I had on my exam, but it's close enough and the context is the same.