Just to clarify, this was at an outward bound centre. When you say your boy was 'lost' and 'the leaders did not know where he was', do you mean that he was on site at the activity centre still or had left the site? If the boys had gone off in pairs, actually, the leaders probably couldn't say exactly where any of them were (except on site,mas that would have been the instruction) until they met up again. And actually, this is the point if the exercise - they get some independence within reasonable bounds for their age, and the leaders purposely do t know their exact pinpointed location for the period of time given. Those children are not 'lost' because they are on site, having freedom within given parameters and rules.
So your boy went off on his own and didn't go with a buddy as told to. What exactly did he do for 2 hours and where was he all that time? Did he see or come across any if the other boys during that time? Did he feel he was lost himself and not know where an adult would be if he needed one?
Scouting at this age, D of E in a couple of years....it's all about freedom and independence within safe parameters. Parents need to understand children will be given instructions about timings, acceptable behaviour (staying with buddy) and how to keep safe, along with what to do if there's a problem. They are told this stuff and then given some freedom to perhaps go round a theme park or an outward bound centre and choose activities, without a leader constantly with them. It is age appropriate for them to have a level of independence. Parents need to understand these activities are not like a school trip for 6 year olds, where an adult remains with the group the whole time. They don't need to.
Yes, these events need to be run carefully, risk assessed and be age appropriate. Even though Scouting or D of E is often run by volunteers, it is not amateurish. The training And paperwork needed to take kids on day trips, never mind residentials is quite staggering. These organisations take it very seriously. Information will be given clearly to the kids. However, it absolutely is the case that the kids need to listen too and follow instructions. This might mean staying within a defined radius, making contact with leaders at certain times, certain safety rules. These rules are also age appropriate and the children involved have to be able to follow them.
Op, perhaps you need to speak to your son a bit more about the importance of listening and doing what he's told, if he's to be trusted to go to something like this again.