There have been plenty of examples over the decades of "Life Beyond The Screen" from various TV shows. That is, mentions of characters and events that are not seen during the actual show, but are mentioned in some way.
The best example of something having happened off-screen in a TV show can actually be found in a movie.....
Out of the airlock steps a tall masked figure.
A moment of suspense as Terrell and Chekov watch, terrified.
The mask is peeled back.
CHEKOV (aloud despite himself) Khan! KHAN is startled by the recognition; comes over and examines Chekov and Terrell. KHAN (to Terrell) I don't know you. (to Chekov)
But you. I never forget a face. Mister Chekov, isn't it? (wonderingly) I never thought to see your face again.[From "Star Trek II: The Wrath of Khan"]
But the thing is, when Khan Noonian Singh showed up in the 'Star Trek' TV episode "Space Seed", Pavel Chekov was nowhere to be seen by the audience. In fact, Walter Koenig had not even been hired yet to play the role.
So we have to accept that the "Khan-Chekov Principle" was something that happened off-screen.
Except for the shows in which succeeding episodes pick up right where the previous one ended, it can be suggested that there is plenty of room between episodes for adventures to happen. In fact, even with those other shows there are plenty of chances between scenes and during the commercial breaks for small moments to happen that could be referred to later. And that was the case with the Khan-Chekov Principle. You must have been in the bathroom when that happened.....
Here are two other examples, from my second favorite TV show of all time:
"IT'S YOUR FUNERAL"
Yet another Number two is retiring to make way for a successor,who will receive the hand-over at a ceremony. Number Six hears that a bomb made by the Watch-maker is to be detonated at the ceremony, killing the retiring Number two and, with the Watch-maker's daughter, goes to warn him. However it turns out that the intended victim is somebody else altogether and their death could spell calamity for the whole village. Number Six must stop this plan.
This episode contains more No. 2s than any other - not only can one see Andre Van Gyseghem's retiring No. 2, and the scheming Derren Nesbitt as his "heir presumptive", but two others - albeit given only a line each.
Here are those two mystery Numbers Two:
Since I was a kid watching this show (I caught most of the summer run on CBS in 1968.), I always assumed that these two actors were meant to represent the Numbers Two played by Leo McKern in 'Chimes Of Big Ben' and Mary Morris in 'Dance Of The Dead'. Not much concern was given over to recasting back then because nobody conceived that there would ever be a future in which there would be a home video market or even syndication. The thinking back then was basically "one and done" with one rerun perhaps. (This is why we kept seeing things like the Ricardos' apartment number and Ethel's maiden name always changing in 'I Love Lucy'.)
So I just figured these appearances were too quick to even bother with hiring Mr. McKern and Ms. Morris and transporting them to the set location.
But that was before I began thinking inside the Box, within the "reality" of Toobworld.
Instead, consider this: these are two new Number Twos who tried - and failed! - in their attempts to break Number Six. Whatever their plots were, we just never got to see them play out.
There are seventeen episodes of 'The Prisoner' that were filmed. But unofficially, there were nineteen attempts to get information... information... information... out of Number Six. We just never got to see two of those plots.
By the way, neither of those actors are identified in the episode's end credits so there's no way I could conflate other roles they played their Numbers Two, either before or after their appearances. (I have done such wish-craft in the past for the Numbers Two played by Derren Nesbitt and Clifford Evans. And with my favorite Number Two, played by Leo McKern, I gave him a few family members. (I'm sure you can guess who I chose for his twin brother!)
(Thanks to Femton Breedley)