Dd1 - late Walker. 2 years old, we are at Xscape with the wider family. We all totally forgot she could walk now, it had been literally a week since she'd started walking. Sat having a chat waiting to go on this sledging thing when suddenly a man appears holding her "is this yours" he asked (my sister, whoops) and turns out, in the few minutes we'd been chatting she'd unclipped her buggy, walked off, into the elevator to the upstairs café. She went from toddling one step, to walking as if she'd been doing it for months, in a day.
Dd2 - in the back garden playing with her big sister. We were in the living room chatting (I see a theme). Back garden was totally secure, fenced, padlocked gate. So no reason to think they could escape, could also hear them playing as all the doors were open. I look up, and a man and woman are walking up my drive way, one holding the youngest and the other her trike. She'd scaled the fence, dropped into the neighbors garden, her sister handed her her trike, and off she went to get some sweeties. She was 2 as well. We lived on a main road, which had HGVs up and down it constantly. The man had stopped his car because she was in the middle of the road. My blood still runs cold now, 7 years later.
Op, no harm, no foul. You've had a reminder, as did I and other PPS, that kids are fucking pricks sometimes and we need eyes, ears and hands everywhere. It does get easier.