Poor kid - it's awful that he had such a scare. I would definitely speak to the principal about how upsetting this incident was to your DS. If his ADHD is disabling enough, then you can try to write into his IEP that for his safety he must be accompanied by an adult at all times. That way, you would have legal recourse if he is left alone again in a classroom.
Our district is similar to Blackcat's (but without incidents of kids actually leaving school grounds)in that even 5-year-old Kindergarteners are allowed to travel the hallways as long as they have a pass. DS used to be the designated errand boy for his class to the school office back in K. Because DS was subject accelerated for math in 2nd grade, he occasionally came back to an empty classroom due to assemblies or other activities. A few times he ended up having to go ask at the school office when he either forgot or didn't pay attention during morning announcements.
