It's very possible to have lice and not have any readily visible nits. You won't see actual bugs unless the infestation is severe - those suckers are fast and as soon as you part the hair they've moved. Some salons will look for you to tell you if she has lice, some won't but some will, it's worth a try. School nurse missed lice on my son, salon stylist found it.
Throw away the plastic comb. It is worthless. Buy a Terminator comb. Comb daily for at least 2 weeks following the directions exactly. Make sure you wash or bag all bedding, stuffed animals, pillow, anything she puts her head on (don't forget the car seats if they're cloth, you can buy spray at the drug store for those).