I will admit that I have a morbid interest in crime and criminal investigations. My brain is happy to think about what nasty things bad guys can get up to. As a fiction writer, that is not only going to be bad, it's going to lean towards pure evil—then take it one step further.
My stories are primarily set in the US, which means I have to spend a significant amount of time getting my head around various aspects of US law and police procedures. With there being differences between federal, state, and country (sometimes even city), it can seriously do the head in.
But the joys of being a fiction writer, I only need to be plausible. Not everything I read about US law will make its way into one of my manuscripts, but some readers might be interested in learning about the technological aspects of US law that I uncovered.
Today, it’s the little quirk associated with looking at an arrestee’s smartphone.
Law enforcement needs a warrant to go through your phone.
Smartphones and the digital age have added a whole level of complexity to privacy and security. I don't think anyone will argue that point. However, legal restrictions regarding the digital age have complicated police activities in ways that we never thought possible.
It turns out that it's illegal for the police to search a cellphone, tablet, or any other smart device for any details regarding a crime without a warrant, even if the cellphone's owner has already been arrested. In 2014, the US Supreme Court held unanimously that police must generally obtain a warrant for such a search.[1 - 2]
Chief Justice John Roberts said :
Modern cell phones are not just another technological convenience. With all they contain and all they may reveal, they hold for many Americans 'the privacies of life'. The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought.
However, while cellphones and other smart devices are now protected, all other items that people carry (such as a paper notebook—yes, some of us still carry paper notebooks everywhere we go) may be looked at without a warrant—after an arrest.
The 2014 ruling puts a whole new spin on those immigration stories where people were stopped at the US border and customs officers insisted on passwords, so they could go through a person's smartphone and Facebook. I could be wrong, but I seem to remember those stories hitting the news back in 2016. If that was the case, then those customs officers were actually performing illegal searches.
A warrant is needed to ping your location from cell towers—most of the time.
In 2018, the US Supreme Court expanded the cellphone data protection to include a cellphone's location history and cellphone tower ping data.[3 - 4] However, exceptions for this one were made when we were talking about terrorist bomb threats and kidnappings. If a person's life is in immediate danger (and it could be proven to be as such), no warrant is required.
Both of these little tidbits made their way into my first crime thriller manuscript. And my main character had fun telling the cops they needed a warrant to go through her phone and tablet without her permission.
There are other random details that I've encountered along the way. Perhaps those will find my blog too.
 Landmark Supreme Court Ruling Protects Cell Phones from Warrantless Searches. The National Law Review (30 Jun, 2014) https://www.natlawreview.com/article/landmark-supreme-court-ruling-protects-cell-phones-warrantless-searches
 Riley v. California. Supreme Court of the United States (25 Jun, 2014) https://www.supremecourt.gov/opinions/13pdf/13-132_8l9c.pdf
 In Ruling on Cellphone Location Data, Supreme Court Makes Statement on Digital Privacy. The New York Times (22 Jun, 2018) https://www.nytimes.com/2018/06/22/us/politics/supreme-court-warrants-cell-phone-privacy.html
 Carpenter v. United States. Supreme Court of the United States (22 Jun, 2018) https://www.supremecourt.gov/opinions/17pdf/16-402_h315.pdf