Don't beat yourself up about it. My friend who's son has it found it purely by chance. It's not often you go stretching all their little fingers out to make sure they straighten fully. My friend just thought that he was holding his thumb that way and noticed it when he was holding something in particular (can't remember what though). If they often wait until 4 then there's no harm done by finding it at 3 instead of at 2 or 1.