It's possible they checked before (and the article suggests they had an inclination where the gene should be) but the resultant backcrosses involved a spontaneous recombination of the relevant fragment. It's possible someone checked the wrong vial in lab, or mislabeled results. It's possible something funny "just happened".
Wet lab is hard, and everyone is chasing the eureka moment.
Wet lab is hard, and everyone is chasing the eureka moment.