From CGI's Archimedes:

Admittedly, this IS Snopes, but currently Snopes is claiming that this (both stories) story is false. Now, how far do you trust that Snopes is providing accurate info?

http://www.snopes.com/guccifer-missing-from-jail-cell/

Snip

Origin:On 2 July 2016, Hillary Clinton was interviewed by the Federal Bureau of Investigation. On the same day, conspiracy site whatdoesitmean.com reported that hacker Marcel Lazăr Lehel (whose online moniker is "Guccifer') had gone missing from his Virginia jail cell.

Lehel has made headlines for breaking into the emails of a number of high-level authorities in the United States and his native Romania. He has also claimed that he hacked into Hillary Clinton's and the Democratic National Committee's e-mail servers.

The former taxi driver is currently awaiting sentencing in the United States on charges related to hacking into private AOL accounts:

An astounding Foreign Intelligence Service (SVR) report circulating in the Kremlin today says that the Romanian hacker Marcel Lehel Lazar (known as Guccifer), who successfully gained access to all of the private and government emails of Hillary Clinton, was discovered missing yesterday by agents from the US Federal Bureau of Investigation (FBI) who were sent to interview him at the US jail he was supposed to be in—and his whereabouts are still unknown.

The claim didn't get very much traction until 5 July 2016, when FBI Director James Comey announced that the Bureau would not be pursuing charges against Hillary Clinton. That announcement kindled interest in the earlier report that Guccifer had mysteriously disappeared from his jail cell ahead of an FBI interrogation.

The rumor originated with a single source: the blogger known by the pen name "Sorcha Faal" of the political conspiracy site whatdoesitmean.com. RationalWiki describes the outlet as "sensational" and "outrageous":

Sorcha Faal is the alleged author of an ongoing series of "reports" published at WhatDoesItMean.com, whose work is of such quality that even other conspiracy nutters don't think much of it.

Each report resembles a news story in its style but usually includes a sensational headline barely related to reality and quotes authoritative high-level Russian sources (such as the Russian Federal Security Service) to support its most outrageous claims. Except for the stuff attributed to unverifiable sources, the reports don't contain much original material. They are usually based on various news items from the mainstream media and/or whatever the clogosphere is currently hyperventilating about, with each item shoehorned into the conspiracy narrative the report is trying to establish.

The fabricated whatdoesitmean.com story about Guccifer's in-custody death was picked up a few days later by the Christian Times Newspaper web site, which is just another online fake news outlet (not to be confused with the legitimate Christian Times newspaper).