The technician’s video clearly shows a pulsing light from Mr. Meade’s phone. It also shows Mr. Meade taking the phone out of his pocket and propping it under his chin, pointed at the stage, without looking at the screen. After putting it back, he glances down quickly, then adjusts its position in his pocket, keeping his torso straight. (Mr. DelGaudio showed the video to The Times but would not allow it to be published.)

Someone who wants to turn a smartphone into a spy cam can find plenty of help. There are night-vision apps, tweaks that let you film while your phone appears to be off, apps that stash files in hidden locations, cameras disguised as phones.

Mr. Meade said that he had no idea why the device in his pocket — which he said was a “completely unmodified” iPhone 6 Plus — was emitting the pulse seen on the infrared monitor. (While some Android phones use infrared laser technology to focus, current iPhones, including the 6 Plus, do not have an infrared transmitter.)

Mr. Meade did say that he uses LED flash alerts, which are designed to be highly noticeable. But Mr. DelGaudio said that he saw no pulse from the stage, and neither did two theater employees who observed Mr. Meade.

In a small theater, during a part of the show when the audience is in “complete blackout,” any ordinary light “would have been visible from a mile away,” said Jake Friedman, Mr. DelGaudio’s manager and one of the show’s co-producers.

Mr. DelGaudio may be an outlier among high-profile magicians in his extreme reluctance to put his work onscreen. Scour YouTube and you’ll find only a few 10-year-old clips of him doing card tricks on Spanish television.

Whatever the truth about what happened the night Mr. Meade visited Mr. DelGaudio’s show, word that other magicians may have surreptitiously filmed him drew a strong response from some fellow performers.