Yes, there are some hidden mining pools. It is difficult to detect them, because they send shares in a P2P network only between pool nodes. For example currently I can earn one block per ~40 days when solo mining and 0.001 coins per ~hour in pool. Mining on-chain blocks is getting more and more difficult, so people are sharing some easier blocks off-chain. Also, since merged mining require no additional computing power, it is profitable to both solo mine and in pool at the same time. Everyone just creates blocks as usual: if it meets on-chain difficulty, it is shared on-chain. If it meets pool difficulty (around 1000x easier now, but it depends on node settings), it is shared off-chain.