Legalized breaking and entering?

A tunnel used to smuggle drugs between the Canadian-American border was discovered by federal agents.

There was a disturbing section in it.

Armed with a delayed notification search warrant, agents entered the U.S. house and planted eavesdropping devices and cameras to watch the concluding months of the tunnel project.

Wtf is a "delayed notification search warrant"? Isn't it legalized breaking and entering? Doesn't this give law enforcement agencies carte blanche over what they can do? If no suspicious activity was observed, the warrant would not have been served, meaning that the people under surveillance would have not been the wiser that their privacy has been intruded upon.