The best argument in favor of suppressing the signaling device is that the officer recovered it as the result of an illegal "seizure" of Al in front of the museum. Since the officer did not have reasonable suspicion to seize Al at that point, all evidence obtained after that encounter is the "fruit of the poisonous tree" and inadmissible against Al. Regardless of how it was obtained, this evidence may be used against the other defendants, who do not have standing to challenge evidence seized from others, even if illegally obtained.
This stop constitutes a seizure according to the U.S. Supreme Court's decision in Terry v. Ohio (1968), which defined a seizure as occurring when an officer "has in some way restrained the liberty of a citizen." This restraint can be by physical force or by show of authority. Here, the officer approached Al in her police car. She directed her flashlight at him in a clear show of authority and she interrogated Al ("What's going on here?"). A reasonable person in Al's situation would have perceived that they were not free to leave, thus creating a seizure for Fourth Amendment purposes.
For the signaling device to be admissible against Al, that seizure had to be lawful, which means Ann had to have a reasonable suspicion that Al was engaged in or had engaged in criminal activity. Merely standing on a street corner does not give rise to reasonable suspicion, as the Court held in Brown v. Texas (1979). In that case, the police claimed reasonable suspicion because Brown had been standing on a street corner in a "high drug problem area," which the Court found insufficient.
The prosecutor will counter that the initial encounter outside the museum did not constitute a "seizure." In California v. Hodari D. (1991), the U.S. Supreme Court defined a seizure as the "laying on of hands or application of physical force to restrain movement." Therefore, no seizure occurred until Ann tackled Al. Regardl...