My borderline ectopic back in 2005 ended with a D&C, not because it was ectopic, but for other reasons involving development. If they have been unable to find anything in the tubes or on the ovaries, I'd seek a second and third opinion via u/s if you have to, it may be that your bubba is just hiding.

Are you sure you have the dates right? I know they usually go by gestational age (date of last menstrual period onwards) but is it possible that implantation was a half a week to a week out? I have just found my sheet from my local hospital which has normal HCG ranges.

Gestational Age HCG Range
3wks 6-71
4wks 10-750
5wks 217-7,138
6wks 158-31,795
7wks 3,697-163,563
8wks 32,065-149,571
9wks 63,803-151,410
10wks 46,509-186,977

But then the levels at 6.5 weeks for my borderline ectopic were a little over 1,900, which is within normal limits.

I suppose it would also depend on the normal length of your cycle, normal luteal phase, and how quickly your kidney's process the hormones too.

I hope that helps a bit