Here is a quote from the Novel Dark Mirror Discribing the Ent-D
....The red alert sirens went, and he was up out of his chair before his communicator even had time to speak. He hurried out of the ready room into the bridge. Everyone looked startled, and Troi, in her seat, looked actively upset. "What is it?" he said to Data. The main screen was showing empty space.
"Nothing now," Data said. "But we have just had a contact--fleeting. The helm took us immediately back out of range, as programmed."
"What was it?"
"Here, Captain." The view on the screen flicked. Same starfield--but there was something in the center of it, very distant, that hadn't been there before: a small steel-gray speck. "Enlarge ten times," Picard said. The speck seemed to leap forward.
It was Enterprise. But not his Enterprise. It was a dark gray, even enlarged, a gunmetal color, cool and unfriendly. The design was overtly the same--the great sloped disk of the primary hull, the nacelles, the secondary hull, all where they should be. But the secondary hull seemed larger; the nacelles were raked farther forward, and lower. The primary hull's curve was deeper and now had a frowning look about it. If ships had expressions, this one had its eyes narrowed. It was a cruel look, and intimidating. Just visible, because of the rake of the primary hull, were the characters ICC-1701-D ISS ENT-- The rest was curved away out of sight.
Picard's heart seized at the sight of it. In a way, he had been hoping that everything that had happened so far might have some other answer. But the hope, he now saw, was in vain. The proof of the problem had come hunting them. He looked around, seeing the same unhappy look on everyone's face--and Troi still looked ashen.