You know the parents best, they may have been just asking as they just haven't seen it before and the little one spends his day with you. I understand why your feeling a little rattled ~ do you feel as though they were accusing?
I have a communication report that I fill out daily and email at end of day and any and all incidents go on it ~ comments like maybe the little one had a fall on the mats or bumped into something. I document everything and also have what I call a boo boo log for any big falls and or bumps or cuts/fevers etc. I will take a copy of it and put it in parents bag if I write on it that night.