> On the night of March 8, 1971, burglars broke into an F.B.I. field office in Media, Pennsylvania, and made off with a cache of top-secret files.
If I recall this story correctly, the burglars left a note on the door that day, saying not to lock it, and apparently the people working there respected that, so when the burglars arrived that night, the door was unlocked.
I think you mixed this up with some other story. Original plan was to pick the lock, but additional high-security lock was unexpectedly installed.
From "The Burglary: The Discovery of J. Edgar Hoover's Secret FBI" book (highly recommended!):
"Acting as though he lived in the building, Forsyth opened the always unlocked front door, walked upstairs to the second floor, and went directly to the FBI office door. He felt slightly nervous but very confident.
His confidence quickly evaporated.
He could not pick the lock in thirty seconds. He could not pick the lock at all.
As he faced the object of his many rounds of picking practice, Forsyth was startled. There were two locks, not one, on the main entrance of the FBI office. One was the simple five-pin tumbler lock that he remembered seeing and was prepared to pick, the same one Bonnie Raines remembered. But now there was a second lock, a much more complex one—a high-security lock that was extremely difficult to pick. Forsyth’s homemade tools were useless on this lock.
[...]
Forsyth arrived at the FBI office about that time. The thirty-second break-in plan now long gone, breaking in now seemed like it would be more like a small demolition job than a swift lock-picking exercise.
[...]
This time, in addition to the set of homemade tools in his briefcase, Forsyth carried a crowbar. It fit fairly well in the deep inside pocket of his used Brooks Brothers overcoat. A long time later, he marveled in amusement at how well the overcoat’s pockets, with their unusual depth, could be adapted to the special storage needs of a burglar. He easily picked the lock on the second door, the one that was not used as an entrance, in thirty seconds. But that was just the beginning. There was a deadbolt near the top of the door. He pulled out his crowbar and, with a quick maneuver, popped it. “I had to do it fast. Otherwise, there would have been a long creaking sound. I figured if you cause a quick bang, if someone hears it, they will think a cat knocked over the trash can in the alley, and it’s all done with.” He wasn’t sure that rationale made sense, but he found it comforting at the time.
Then he pushed on the door. It wouldn’t move. Yes, Bonnie Raines was right. He didn’t remember exactly what she had said blocked the door; he just realized now that something “very big and very heavy” was on the other side. To move it even a tiny fraction of an inch, he had to lean on it with all his might. An agent who worked at the Media office said years later that, given the weight of the large cabinet leaning against that door, all of the burglars must have pushed the door open together. In fact, Forsyth alone did it, but with a great deal of worry. “It was obvious,” he recalled, “that if that sucker hit the floor it was going to wake up the whole neighborhood, not just the caretaker who lived directly below the office."
Found the story about door note (quote from the same book):
"As burglars, they used some unusual techniques, ones Davidon enjoyed recalling years later, such as what some of them did in 1970 at a draft board office in Delaware. During their casing, they had noticed that the interior door that opened to the draft board office was always locked. There was no padlock to replace, as they had done at a draft board raid in Philadelphia a few months earlier, and no one in the group was able to pick the lock. The break-in technique they settled on at that office must be unique in the annals of burglary. Several hours before the burglary was to take place, one of them wrote a note and tacked it to the door they wanted to enter: “Please don’t lock this door tonight.” Sure enough, when the burglars arrived that night, someone had obediently left the door unlocked. The burglars entered the office with ease, stole the Selective Service records, and left."
If I recall this story correctly, the burglars left a note on the door that day, saying not to lock it, and apparently the people working there respected that, so when the burglars arrived that night, the door was unlocked.