Investigators looking at phone contents may have conducted an illegal search.

A recent case that had it’s beginnings back in 2014 ended up with a decision by the Supreme Court that police need a warrant to search someone’s cell phone with making an arrest.
Riley v. California, was already considered a major privacy victory but in Illinois it was interpreted by a federal court to mean that opening a phone to look at the screen qualifies as a search and requires a warrant.
The Illinois case involved a police operation where an alleged drug dealer was also charged with possession of an AK-47. An officer testified that while interrogating the suspect, he pulled out a flip phone and opened it. The wallpaper of the phone showed a picture of the rifle. The phone was then used as a foundation for a search warrant of his phone and the metadata about when and where the photo was taken. Despite the officers’ claim that he opened the phone only to turn it off, the court ruled that police have no right to open the phone and look at the screen without first getting a warrant, even if it’s only to turn the phone off.
The officers’ opening of the phone amounted to more than a “cursory inspection” because it exposed contents of the phone (the picture on the wallpaper) as the officer had to manipulate the phone to view the image and the image was not considered to be in ‘plain view’.
Although there are exceptions for ‘exigent circumstances’ that allow for a search without a warrant, they are usually related to issues of officer safety or preservation of evidence. As neither one of these issues were a part of this search the search of the phone violated the Fourth Amendment prohibition against unreasonable searches and seizures.