Robin, please don't call me "B.M." In the medical community, that is the common abbreviation for "bowel movement."
grooves on a swingin' happenin' scene, baby!
This seems to be a situation where the artist didn't read the script very carefully. Usually, Gordon turns around and B and R are just gone. In this case, he turns around and sees them running out the door. Not very mysterious, to be sure, but we appreciate Gordon playing along.
See you tomorrow!