I'd have probably called the cops.

That is serious neglect.

Ugh, you are absolutely right to inform the parents. Thank goodness nothing happened but it so easily could have.

I would have been furious as well. 3-years-old is much too young to be outside unattended, especially in the front yard.

What did you say to this nanny when she finally appeared?

Normally I am not one to get in the middle of another nanny/family situation, but in this instance I would. Like you I would talk to my MB and let her know that I am not comfortable doing drop-off playdates with this nanny as the adult in charger. Then I would ask her if she'd like to contact the other MB or if she'd like me to contact her directly to share what I found when I arrived at her house.

I feel sorry for the other child if this is an indication of the type of care that nanny is providing.

My 3 year old charge was on a play date this morning with his little friend and the friend's nanny. For the past 6/7 mos both families have done drop off play dates which has worked out beautifully. Until today. I was about 10 min early to pick up my 3yo charge and he and his 3yo buddy were outside, in the driveway, alone. When I rounded that corner and saw "my" kid unattended no lie my heart practically jumped from my throat. Anyway, it took 5 full minutes of me standing there talking to my charge (he at this time had gone into the converted garage to get shoes- he was shoeless outside) before this other nanny even came to see what was going on. I sent my charge charge to get the nanny and he came back and said " she said no".
Anyway, she left two 3yo outside alone. On a busy street. I'm telling my MB and this other child's mother what happened. In the meantime I want to punch this other nanny in the face for being so careless.