They say "hindsight is 20-20." I don't think so here.
Pick any character and you can probably make an argument that it was "obvious."

Sirius? Lupin (if you stopped reading before the end of HBP)? McGonagall? Any of the teachers?
Any shopkeeper? Any Ministry official? Any side character who didn't appear to be married.
It could work for anyone if you tried hard enough.