How does a Muslim fall into subtle polytheism (shirk khafi) unknowingly, especially mosque imams?

Hidden polytheism (shirk khafi) is ostentation (riya'), and the Prophet, peace and blessings be upon him, called it minor polytheism (shirk asghar). On the authority of Abu Sa'id al-Khudri, the Prophet, peace and blessings be upon him, said: "Hidden polytheism is when a man stands to pray and beautifies his prayer because he sees someone looking at him." The reason it is called by this name is that it does not appear to people as polytheism; rather, it appears to them as righteousness. The Prophet, peace and and blessings be upon him, described it as "more subtle than the creeping of an ant." To guard against it, we say: "O Allah, we seek refuge in You from associating anything with You knowingly, and we seek Your forgiveness for what we do not know."