I would not accept that he 'forgot' that he was not supposed to do that. I hope you got very very very cross with him that he did that. He is old enough to know the rules.
Does he walk to/from school every day ?
Why did he go to the Kita ? What was his 'incentive' from the child's point of view ? Is it fun ? Did he go to Kita to play ? Does he go to Kita at other times of the day ?
So I guess that in your situation, I would try to find out if maybe something has been happening at the Kindergarten to cause him to run away (for example, maybe another child bullying him?).
I noticed that a couple years after our son left the Kindergarten, they finally put up a fence around it, with a locked gate so that the kids couldn't "escape" (and perhaps also so that no one could easily come in). I do think it's the Kindergarten's responsibility to ensure that the children feel comfortable there and do not leave or cannot leave easily, to whatever extent possible. The kids are their responsibility while they are there.
(Not saying this is good behaviour but at that age, they have primal insticts and if they are not well they are bound to follow them)
So like Pancakes I would encourage you to understand what is at the root of the behaviour happening.
Stay strong - this too shall pass.
Of course it is the teachers responsibility to safeguard the children in their care. But normally 5.5 year olds don’t need monitoring to ensure they stay put. They are usually so involved in their day that it is not something that would suggest itself to them.
So you definitely need to figure out what is going on.