The man and the woman arrived at the Washington steakhouse one evening in May 2019 and took a table on the patio, near where five diners already were seated and seemed to be enjoying themselves. Jackets were off, laughter was heard and wine was being poured. A cigarette dangled from one man’s hand.

But there was something else that stood out: Among the five were Senator Robert Menendez of New Jersey and his future wife, Nadine Menendez.

The couple who had arrived late appeared to be on a date. She was wearing a black dress, and during their meal he moved around their table and used a camera to, it appeared, snap her picture.

But in reality, court testimony on Tuesday showed, he was photographing the people behind her, seated at the senator’s table. The couple were members of an F.B.I. surveillance team, and they had deliberately sat near Mr. Menendez and his group, which also included an Egyptian government official.

The undercover F.B.I. investigators also used concealed cameras to photograph and video-record the people at the table. The female investigator, Terrie Williams-Thompson, testified that at one point, she overheard Ms. Menendez, referring to the senator, ask the group, “What else can the love of my life do for you?”

The extraordinary scene of a U.S. senator under F.B.I. surveillance in an upscale restaurant, Morton’s The Steakhouse, not far from the Capitol was detailed by Ms. Williams-Thompson in testimony at Mr. Menendez’s federal corruption trial in Manhattan.