I will need to dig to find it, but I seem to recall within the past month seeing an article that laid out the decision-making process and timeline that led to Comey reopening Hillary’s case in September 2016. It shouldn’t be too hard to pin down who would have tipped Weiner off. The Bureau had previously given Hillary plenty of advance notice to review her files during the Benghazi phase of the investigation.
Also see this: "Other law enforcement officials, however, have said they are concerned that the issue seemed to die for a period of time at McCabe's desk, without explanation. . .The dormant laptop issue then appeared to gain new attention inside the FBI and Justice Department. At a meeting of senior officials of both agencies, senior Justice Department official George Toscas asked about the status of the inquiry into the emails on Weiner's laptop, according to people familiar with the matter.": Internal Justice Department probe eyes McCabe’s role in final weeks of 2016 election