In the season 4 opener "Herrenvolk", X's position as an informant is discovered by the Syndicate. When suspicion arises after the finding of photographs that were taken of The Smoking Man (William B. Davis) by X, false information is planted at the First Elder's behest, in order to root out the leak.
In the season 4 premiere episode "Herrenvolk", X's position as an informant is exposed, and false information is planted at the First Elder's behest to identify the source of a leak.