It’s possible that a seizure is preceded by a chemical change that the dog can smell. I think dogs can detect diabetes because diabetics generate acetone or another ketone in their blood.
It is amazing what some dogs can do. Mine is mostly good at playing beach ball soccer and barking.
I watched a program last nite - Animal Miracles, I think - where a police bloodhound was able to track a baby that had been kidnapped and placed in a pickup truck by air scent tracking.
Over the course of a couple of hours, I take it, the police, led by the dog, located the baby 20 miles from where it was taken - freeway on and off ramps, California metro streets.
When the dog led them to the freeway, they would pick up the dog and take it to the next exit (so the dog didn’t have to walk the whole 20 miles) Amazing!!