Has anyone had a positive pregnancy test during implantation?

No is the short answer since there is no hCG yet to measure.

How would that even be possible?

hCG causes a "positive" test. hCG isn't produced until after implantation. You can't test positive for something that isn't produced yet.

No.
That's not how it works

That is simply impossible.

You might be able to detect it a few days later with a BLOOD test.

A few days after implantation is the earliest you could get a BFP. My second pregnancy was from IVF, and since the embryo is placed directly into the uterus, implantation occurs within 24-48 hours. My first *very* faint BFP was 5 days after the transfer. I've heard of women get BFPs 4 days post-transfer, or even more rarely 3 days after, but that's not very common.

No. Wait at least a week or until your period is due.