Yes, I agree mostly, what I wrote was my theory before I read about the dogs not being able to track her scent.
There was an article with mention of an eyewhitness report though, she was seen coming outside her home and going to the car "with something red in her hands". I found that intereseting, but it doesn't really tell us much.
It's strange, we don't know. If she was the victim of the bloody attack inside her own home, she was the one most likely bleeding. It could be whoever saw her was seeing the killer forcing her out to the car to take her away. The scent dogs did find her scent coming out of her house to her driveway. If you look at photos of the house, there was shrubbery and trees in the front yard and along the driveway. It's possible whomever thought they saw her with something red only got a partial view of Joan, didn't see the person who was taking her away.
IIRC, LE scent dogs traced the trips she made to go across the street to the neighbors to drop off the kids, then traced back home. Then there was another scent trail that came out of her house and ended in the driveway. That's why police concluded that she left in a car. They took the dogs up and down the street outside her house and the road nearby, but could find no scent.
They also checked the back yard and all the woods behind her house, but also found no scent.