There is so much on the internet and news about daycare and if you can really know what your child goes through. And all of those articles say exactly this - arrive early, peek in, watch your caregiver in action with your child from afar etc. He may very well be watching to see how she treats the child - ignores, seems to cater to others over his, is a yeller or screamer, pays very little attention to the kids, etc. It is about trust but trust is earned. A smiley face at drop off and pick up is not enough for some. Child may be rather new or he may be going through a new stage and crying or putting up a fuss about daycare - not because there is anything wrong with daycare but because the child wants to be with the parent. The parent needs reassurance.
I would probably ignore the parent while at the park but cut my visit shorter than normal just cause it would feel weird. Then confront the parent next pick up saying something like you are really lucky I saw you at the park yesterday because you scared the providers there who are trained to be extra vigilent when we take the kids out in public. The police were called reporting you as lurking and stalking the children at the park. I recognized you and diffused the situation so they didn't come - you were lucky. I get that you wanted to watch your son with his friends but you really need to think twice about stalking behaviour in this day and age.
What it does is acknowledges that you saw him, explains the police as a way to make him think twice about doing it again, assures him you are vigilent to dangers, etc. At the same time it didn't accuse him of anything untoward or make it about the provider and trust but still gets the message across.