I also had the school call me and tell me my son wasn't there and wanted to know why he was absent. I was at work and might have freaked out if I hadn't seen him actually walk in the building that day. Sure enough, he was there, he was just behind the wall where they put their backpacks when the teacher was taking attendance. The school secretary was so apologetic when she called back to let me know he was there. I think I scared her more than she scared me!
One time my son did get away from me. He was 3 and a half and we were at my sister's high school graduation. This was a perfect storm of problems. The football stadium was extremely crowded, it was hot, the stands were packed full, and my son wanted to run up and down the bleachers instead of being quiet. I managed to keep him wrangled until they called my sister and then I took him and went to leave. I had to get the stroller and of course, my son didn't want to get in it at all. So I'm trying to hold his hand and push the stroller with my other hand without bumping into the people who were standing and walking about. Next thing I know, he's slipped out of my hand and is running off and I lose sight of him in about two seconds. I threw the stroller and my purse to the side and took off after him and couldn't find him for about three minutes. Longest three minutes of my life! Luckily, he stayed in the gated area and didn't get out at the exit, because then he would have had access to the parking lot and streets. Also lucky that my parents live in a safe town since my purse was still there when I went back.