might be like a kiddy teaching grandma to suck eggs but i can see your concern about this.
I would think that as long as you phrase your letter right there can be no comeback, so i would write a letter saying a needle wasn't seen.
I would also explain in the letter, that as you are not covered or qualified to handle this type of waste, a syringe being found would have resulted in the affected area being left until you had contacted head office first as this is part of your h&s policy.
This type of waste would then be dealt with by a qualified person in agreement with head office.
good luck