I've always said goodbye and give her a kiss. I've already told her where I'm going and when I'll be back but when I give her a kiss I'll tell her again if she seems concerned.
The other parents in LO 2 year old preschool all sneak out and both DH and my parents always try to shoo me away if they're babysitting and I go over to say goodbye.
It seems to work better for her though to have that time where she says bye bye but it seems like that's not the case for most kids.