Four empty frames have hung on the walls of the Dutch Room on the second floor of the Isabella Stewart Gardner Museum since the infamous, still-unsolved heist 26 years ago. They serve as a placeholder until the art is returned, and as a reminder—starting conversations among the museum’s visitors. The empty, opulent gold frames have become iconic themselves.

There is now a fifth empty frame on the walls of the museum, this one in the Blue Room on the first floor. It is the frame that belongs to Édouard Manet’s Chez Tortoni, and it has recently been hung in its rightful place all these 26 years later.

According to Anthony Amore, security director at the Gardner Museum, the frame was put on display as a way to increase awareness of the missing artwork. “One of the things that we accomplish by putting the frame up is that people get a sense of the size of it,” Amore said. “I think many people don’t understand the different dimensions of the things, and anything we can do to improve that is worthwhile.” Chez Tortoni measures just about 10 by 13 inches.

“I have found, incredibly, that very few people in the Boston area and, in fact, in the law enforcement realm, have [any] idea what our images look like,” Amore said. “They’re so unfamiliar with the stolen pieces that it’s stunning to me. And that worries me a great deal.”

With the artwork still missing, it is vital that the images of the pieces remain familiar to all of us. As Amore pointed out in his fascinating book, Stealing Rembrandts, most stolen artwork is recovered either immediately or one generation later. It is now “one generation later.”

Of the 13 stolen pieces—now collectively valued at $500 million—the theft of Chez Tortoni remains the biggest mystery of the entire big mystery.

While the thieves took the tapes of the security footage with them, there are printouts of the motion sensor activity that give authorities a pretty clear idea of the movement of that evening.

Here’s where it gets weirder: The motion sensor records do not show any movement in or near the Blue Room, the location of Chez Tortoni, from the time that the thieves entered the museum to the time that they left. In fact, there are gaps in the motion sensors for two 20-minute periods of time: 20 minutes before the thieves went upstairs to the Dutch Room and began stealing things, and 20 minutes before they left. Of the 81 minutes that the thieves spent inside the museum, about 40 minutes are unaccounted for.

The only movement detected in the Blue Room occurred shortly before the thieves entered the building. According to Amore, this movement was consistent with one of the guards making his rounds. That the only movement recorded in the Blue Room belongs to one of the guards has led to a great deal of speculation about his involvement.

Richard Abath was one of the guards on duty that evening, and it was Abath that actually broke protocol and buzzed the thieves—dressed as Boston police officers—into the museum. Both guards were tied up in the basement and discovered by police the next morning.

Last August, the Gardner Museum released never-before-seen security footage from the night before the heist. The video, taken just before 1 am, shows a vehicle pulling up to the side entrance of the museum. A man walks from the car to the entrance and is buzzed in by a security guard. Once inside the museum, the man is shown standing at the security desk. The guard on duty, and shown clearly in the video, is Richard Abath.

Some have interpreted this as a possible dry run of the crime. Regardless of what it was, it doesn’t look good for Abath, who has always maintained that he did not know anything about the crime. “We don’t know what we don’t know,” said Amore. “We have nothing to point at to say that that he was involved.”

Adding to the mystery of the theft of Chez Tortoni is that the frame was not left on the gallery floor like the other frames were. Instead, it was left on the security director’s chair, which seems like a very personal jab.

“Everything about the Chez Tortoni is really mysterious,” said Amore. “If nothing else were stolen the night of the heist, we’d still be talking about one of the biggest art heists in history.”

Chez Tortoni