It is easy to have them, to do a cursory comb through, and not find anything. I know this from personal expeience.
At a time in my life when I had no children I had no reason to suspect I had headlice. I went to the doctor as I had itching on the back of my neck and my shoulders and a rash. Funnily enough the itching wasnt in my scalp. I had asked my husband and another female relative to look in my hair, they found nothing. The doctor said it was a reaction to one of the lotions I was putting in my hair, possibly styling mousse or conditioner. He told me to wash my hair with very plain shampoo and use nothing else.
This went on for a while, then one day I found something suspicious in my hairbrush. I thought at first it was a small insect. It then occurred to me that it might be headlice, I bought over the counter treatment, treated all my immediate family. Result: no more itching on the neck or rash on the shoulders.
Since the children joined us, we have had an infestation, and it took me quite a lot of careful combing, wiping each sweep of the comb on tissue paper, and looking at suspicious specks with a magnifying glass in good fluorescent light to identify them. We used the malathion twice, and now comb with conditioner, and nitty gritty comb, and spray before school with nitty gritty repellant lotion.
It is really easy to do the combing half arsed and not find anything then live in complacency. It is easy to get someone to look in your hair and find nothing unless the special conditioner, comb, wipe, inspect regime is used. But you have to keep on top of it as it feels vile when you get them yourself, and the itching DID keep me awake at night!
So do everyone a favour and get the comb and conditioner out, even if there are no symptoms, or you havent seen anything.