Yep sometimes before the tiny flag emerges, you'll get a leaf thats about half the size of prior leaves. It's actually pretty easy to miss.
When you just notice a flag leaf on a plant, as you celebrate, look at the leaf before it. There's a good chance that it was smaller than the one still before that one, that you may have not noticed as it happened. It's easy to miss.
Anyway, congrats once again! Keep us posted!