Their sensors don't appear to work well against ships at warp while at sublight since "The Picard Maneuver" is a viable tactic.
An interesting point. It seems that special observatories/listening posts, pointing in the right direction, are needed. The Dominion and Breen were able to go right around and through Federation space to get not only to Federation territory (like Earth), but also to Romulan and Klingon space, which happens to be on the other side of the Federation from Cardassia - all the while keeping their supply lines and the like. If they were easily detected, they could easily be intercepted or prepared for.