“turned back from the carved idols.” It is likely that Ehud acted as if he had received an oracle (a revelation) from the idol gods, and that made Eglon willing to listen to his “secret message.”
“that were near Gilgal.” This was early in the Judges period, and so not long after the death of Joshua (likely less than 75 years), and yet already Gilgal, which had been Joshua’s headquarters for years, had now become a place of pagan worship. The Devil is aggressive and persistent, and he works to promulgate ungodliness in every place and in every aspect of life. If believers are not diligent to be and stay godly, the Devil will always replace the godly with the ungodly, and sooner rather than later.
“The king said.” The Hebrew text just has the pronoun “he,” but it refers to the king.
“And all who were standing by him went out from him.” This was the work of God and as good as Ehud could have hoped for. He could not have made this happen, only prayed that it would happen. It turned what otherwise would likely have been a suicide mission into a great victory for God.