Abdominal ultrasound at 7w3d showed empty sac. hcg rising normally, is there any chance that the embryo was "hiding"?

Sounds odd: Usually by 7 weeks you should see something but is it a 7 weeks sized sac or is that based on your lmp. If its a smaller sac, maybe your dates are off a bit and then there's a chance. Next would want to make sure that the sac definitely satisfies criteria for an intrauterine pregnancy or else an ectopic pregnancy needs to be at least thought about.