Theodoric Meyer and Peter Maass of ProPublica, the public interest journalism group, have written a useful and readable guide to the many ways the government can legally snoop on your online data without a search warrant.
From their introduction: The U.S. government isn’t allowed to wiretap American citizens without a warrant from a judge. But there are plenty of legal ways for law enforcement, from the local sheriff to the FBI, to snoop on the digital trails you create every day. Authorities can often obtain your emails and texts by going to Google or AT&T with a simple subpoena. Usually you won’t even be notified.The Senate last week took a step toward updating privacy protection for emails, but it’s likely the issue will be kicked to the next Congress. Here’s how police can track you without a warrant.
Their report, No Warrant, No Problem: How The Government Can Still Get Your Digital Data is available online: https://www.propublica.org/special/no-warrant-no-problem-how-the-government-can-still-get-your-digital-data