Don't beat yourself up, if anything it's a bit of a reality check: how quickly they can disappear :-/
I took my three kids 6,3 and 2 for a walk around a lake, 3.6kms to do one lap. Dd 1 (6) was riding her bike, dd2 (3) riding her bike and ds (2) in pram. Dd1 was going ahead a bit and then stopping and waiting for us to catch up. This was fine, I had told her she could go a little ahead but then had to wait for us. We got half way around and I could no longer see her! There were some slight bends so I kept thinking she'd just be around the next bend waiting for us. No she wasn't so by this point I said to dd2 she had to ride fast as we had to find dd1 so I started jogging but had to slow down for dd2. I didn't know what to do! There were quite a few people traveling in the opposite direction so I asked a lady if she had seen Dd1, the look on her face was absolute fear until she then said yes yes I did see her!! Anyway it felt like forever but we got to the end, which is a park and I *knew* she would be there... She was sitting on a swing. I couldn't get too close because I was so relieved but totally furious! I called out to her and she started running over, happily, until she saw my face!!! Through my clenched teeth I told her to go get her bike and get in the car and think about why I was so mad! She understood. I silently cried on the way home.
My other most recent event happened at dd1 school. I had my two little ones with me as it was a special morning tea thing on. I had them both at my side and was making a coffee talking to another mum. I finished making it and turned around and said to dd2 where's your brother? As she said she didn't know another mum pointed up towards the car park and as I turned to look another mum was walking him back down to me. As I got to her she said he had run onto the road....in front of a car!!😳 I felt like s%*t :-( I am normally so safety conscious, friends even pay me out about it!! But in the 30 seconds to a min I was distracted he ran!!
Thankfully dd1 school is very small and everyone drives very slowly through the car park, also the mother who ran after him has her own young son who used to be quite the runner and she is very fast ! ! I felt crap that I had let him get away but I didn't have the fear part as I didn't even know he was missing or in the car park until it was over.
Hope you're not feeling too bad!!!