Eagle-eyed observers claim to have spotted Clinton, wearing what appears to be an hidden earpiece during the debate, Daily Mirror reports.
Footage from the 90-minute discussion at Hofstra University in New York also appears to show the outline of wires and a battery pack under Clinton's jacket.
Catholic Online website asked: "What is it for? Is it so a team can feed her answers? Is it an anti-seizure device? A hearing aid?"
Social media users were also quick to point out that Republican Trump was not wearing a similar device.Following criticism of his debate performance, Trump claimed he was given a "very bad" microphone and that debate moderator, Lester Holt of NBC News, asked him "very unfair questions".
He told Fox News:
"I don't want to believe in conspiracy theories, of course, but it was much lower than hers and it was crackling."In response, Clinton told reporters on her campaign plane: "Anyone who complains about the microphone is not having a good night."Fueling earlier conspiracy theories that Clinton is hiding a secret illness, some observers suggested the mystery box was related to her alleged ill-health.