REW masks all color genes, it is not a replacement for the color genes, both parents have color genes hidden and you can figure that out by the pedigree.
Draw up a simplee punnet square. In with REW being recessive, both parents REW can only produce REW.
The only way I can think of is an accidental breeding, or a kit getting through the wire of a neighboring cage - sounds improbable. I've heard of it, but third-hand so I'm not convinced it's true.
I love a good mystery - can't wait to see what you come up with.
Some background info for this challenge.
Mother's pedigree says her parents were: D=seal, S=sable point.
Father's pedigree says all REW.
Does are on one side of the barn, bucks on the other.
There are wooden slats between the cages.
Once they were done, into their respective cages they went.
Do you own any other harlequins, or rabbits with harlie in their background?
I can think of three possibilities, all of which seem a bit far-fetched:
1) Doe bred with another buck through the wire (I've had this happen, though not when a doe was with a buck at the time)
2) Doe retained an embryo from a previous breeding, and it implanted at the same time and grew with this litter (I told you it was far-fetched! Rabbits can do delayed implantation, but it isn't usually just one!)
3) Doe is actually a BEW/himi, and just looks like a REW.
Of course, if someone is playing put-and-take with your nestboxes and just didn't tell you . . .
ETA: A seal is a rabbit with 2 shaded genes in the C series, so a seal cannot have a REW baby. Somebody messed up on your doe's pedigree!