Police have a warrant to arrest Terry for murder. He is one of two current suspects in a Homicide. They plan to arrest him at his home, which is a two story house. When serving the warrant, Terry is found upstairs, and he surrenders peacefully. Police go downstairs and open two closet doors. One of the closet doors opened reveals a sawed-off shotgun, the suggested murder weapon. One of the officers proceeds to open a downstairs desk drawer, and observes and seizes a disguise, a passport, and a plane ticket to Argentina. There is no search warrant. What, if any, evidence is admissible against Terry? Why?

Is this homework?

The officers have just screwed up any case the prosecution has against Terry (cf. John Grisham).

There was an arrest warrant,

And this is not clear cut, ( but your text book will act like it is)
Most likely the items in the closet will be allowed in, since they may search the house for the other suspect or for others, for officer safety, so they may search in any area large enough for a person to be at.