"Then he said to me, 'Son of man, look toward the north.' So I looked, and in the entrance north of the gate of the altar I saw this idol of jealousy. And he said to me, 'Son of man, do you see what they are doing - the utterly detestable things that will drive me far from my sanctuary? But you will see things that are even more detestable.'" NIV translation
What was this "idol of jealousy"? In 2 Kings 21, we see that Manasseh had set up an Asherah pole in the temple which was removed by Josiah. This idol is probably that Asherah pole that they had kept and now brought out like Manasseh had done before.
"Then he brought me to the entrance to the court. I looked, and I saw a hole in the wall. He said to me, 'Son of man, now dig into the wall.' So I dug into the wall and saw a doorway there. And he said to me, 'Go in and see the wicked and detestable things they are doing here.' So I went in and looked, and I saw portrayed all over the walls all kinds of crawling things and unclean animals and all the idols of Israel." NIV translation
Now, Ezekiel is taken to a basement area where there were all kinds of pictures of animals and idols on the wall. This would have been like a secret sanctuary but, as we see, God knew that it was there even though the common people may not have.
"In front of them stood seventy elders of Israel, and Jaazaniah son of Shaphan was standing among them. Each had a censer in his hand, and a fragrant cloud of incense was rising. He said to me, 'Son of man, have you seen what the elders of Israel are doing in the darkness, each at the shrine of his own idol? They say, "The Lord does not see us; the Lord has forsaken the land."' Again, he said, 'You will see them doing things that are even more detestable.'" NIV translation
We see that the leaders of Israel were in this little secret sanctuary bowing down to idols. These were the same elders that were sitting with Ezekiel in his house. They were saying in their hearts that they would replace God with all kinds of animal gods because He had abandoned them.