I asked my missus about this, 'cos she regularly gives blood.
She's had a red ticket put on her chair before.
It's to inform the staff to keep an eye on that person.
My missus went to give blood once and she went dizzy, so the next time she went, they placed a red ticket on her chair as a marker, so that the staff could keep a more watchful eye on her.
I know that you said your daughter had never given blood before, but maybe there was something that prompted the staff to give her a red ticket - medical history maybe .... or maybe she was just nervous ??