It depends, but usually after 7 weeks it is visible in an abdominal scan.
If she is less than that, they will need to do a transvaginal scan to see bubs.

If it is in her tubes, a decent technician would have looked for it. The dating scan should check the ovaries, tubes and the uterus to make sure all is well and normal.
However, if she DIDN'T have a TVS then chances are the tech would have missed the sac.

If her blood test comes back positive, she will need to go for another proper scan and find out what is going on. GL!