I'd be curious to know how long that child had been in the caregivers care and info about the location/timing. Did the dad work nearby and happen to be on lunch break and was trying to peek at his child? Or did the dad take time off work and drive out of his way to watch?
There is a strong chance this dad was trying to catch a glimpse of his child and was not watching the teacher. Maybe the kid is a pain in the butt at home and he can't believe the kid behaves at daycare and wanted to see it for himself. The caregiver can't know what the dad was thinking unless she asks directly.
I admit that a while back we were on vacation at a resort that had an attached daycare and we left our 18month old for short spurts while we had couple time. And...we ourselves peeked in at our daughter as she played outside. It was the first time she was ever left in a daycare like setting without me with her and we both were very curious on how she behaved. We had zero care as to what the daycare 'teachers' (a bunch of teens) were doing we wanted to see her in action. Do we know we looked like fools? For sure. The daycare was smack in the centre of the resort so all the activities my husband and I attended were more of less in view (if we hopped up on a bench or peeked behind some trees etc). So throughout the days we would peek and then go about our business.
For us, it had nothing to do with mistrust and had everything to do with parental curiousness. We had the rare occurrence to see what our child is like when we are not around.