Iowa State walk-on QB Michael Frankl threw an 80 yard touchdown pass the first play of his senior season. He sprinted downfield and chest bumped his receiver in the endzone and landed funny. After limping off the field, he was evaluated by the trainers (Iowa State students) and given the ok to play. His first play the very next possession, he attempted to scramble but essentially fell over, got sacked, and from there it was determined that he had a torn ACL.
I'm no doctor, but I know about this instance and I also was a coach for a young lady who had this exact same issue. Three instances in which trainers were unable to identify an ACL tear until after the individuals were cleared to play.
I don't know how ACL's work but I'm assuming its possible for there to be a tear before it becomes loose and before a pop is heard.