It doesn't matter if you're at work or not, if you're a mandated reporter, you have to report.
In NYS, teachers and RN's are mandated child abuse reporters. About 10 or 12 years ago, there was a little boy - maybe 4 or 5 months old - who was shaken to death by his mother's boyfriend while mom was at the library. I won't go into the boring details (I did a rant on it back when it happened), but I will tell you that the woman who was working with me at the library and I spent a couple of weeks sweating bullets, because she's a teacher, and I'm an RN, and we were both terrified that we'd missed something that we should have reported when mom would bring the baby in - even though we were working as library clerks at the time of the encounters.