GUYS--what would you think about this?? IS This FWB or something more serious...Seriously???

This guy friend and girl friend slept together after being friends for 2 yrs. guy said no attachment however, now when they see each other they look into each others eyes, they talk a lot more, they flirt more, they bump into each other on purpose, they make plans to see each other in the future (with no mentioned- he said when it happens it will just happen). Is this a FWB or becoming more serious?