I have to agree... it's unlikely in the extreme that it was lice. (Glitter, cake, gum, glue, sand, I can't even list all the things I've pulled out of my son's hair). And if it was, ditto, a mom bringing in a kid who's got the sniffles to a public event is more dangerous than a mom bringing in a kid who has lice. Lice is annoying, but not as much of a health concern as the common cold, much less all of the other bugs that go around.
I do have to say though, that especially during lice scares (and working in hospitals means that you have double the number as regular parents, because patients bring them in all the time), I get a little compulsive about checking my son's hair. And then there is the inevitable "find"... glitter, playdoh, carpet fuzzy, blade of grass... and he washes his hair every morning. We've yet to have lice, but that doesn't stop me from not even realizing that I'm checking. It's an annoying habit of healthcare workers, actually, the checking hair, lymph nodes, breath, foreheads... 99% of the time our kids our completely fine... but we do it all the time. And it's almost a knee jerk reaction if someone else happens to mention "x" is going around and our kids are close by. <laughing> NOTHING sets me to casually examining my son's hair (forehead) etc., than a casual mention in conversation of "x", regardless of whether we're in public or not.
As to reporting it to her work... you COULD... but telling her boss that she appeared to be checking her son for lice in a public place... I don't see what it would do. It's certainly not against medical ethics. If anything, checking kids for lice off duty, is above and beyond.
You seem to be very concerned and caring about what is going on in your son's school, not just for his sake but for others... and for that kudos. This is most likely one of those non-issues though.
:)
Z