What does Q do in class concerning the night with Margo and why does he wonder about her?

While Q is in class, he makes a note of his favorite moments with Margo. At lunch, Ben and Q go to Ben's car "RHAPAW" or Ben's Buick "Rode Hard and Put Away Wet." Q tells Ben everything. Margo's car is nowhere to be found. Q wonders whether she has departed on another adventure