They put him in custody, because there was no clear instruction about what should be done to him.
When Moses’ father-in-law saw all that he was doing for the people, he said, “What is this that you are doing for the people? Why are you sitting by yourself, and all the people stand around you from morning until evening?”
Moses said to his father-in-law, “Because the people come to me to inquire of God.
So they placed him in custody until they were able to make a clear legal decision for themselves based on words from the mouth of the Lord.
Those who found him gathering wood brought him to Moses and Aaron and to the whole community.
So Moses said to them, “Remain here and I will hear what the Lord will command concerning you.”