When did "Easter egg" begin to mean "hidden feature"?

Can anybody trace the origins of 'Easter egg' for this meaning?

The OED has this sense of Easter egg from 1987:

1987 Re: Does Set Startup work Anymore? in comp.sys.mac (Usenet newsgroup) 13 Nov., The Option-Command ‘About MultiFinder’ easter egg was a good laugh.

This and other early uses were hidden features in software. By 2009, it was being used for hidden clues in television shows:

2009 Wall St. Jrnl. 18 Sept. w7/5 Shows like ‘Damages’, ‘Lost’ and ‘24’ drop clues, or ‘Easter eggs’, as TV writers call them, into episodes that flash on screen for mere seconds.

I found an earlier example of the hidden software feature in Usenet, also in a Mac group.

25th April 1986, Re: a few more option-keys tips in net.micro.mac:

So many of these neat messages seem to come by, but I can't tack them all up (very neatly) near the Mac. So here's a request to all you hardcore MacFreaks out there (and I know you're out there!) -

Why not have a periodically updated list of these option, command, and other combination Easter egg type-of-things?

If you already have one, please, please post it to the net. If you only have a few, please mail them to me at the address below, and when it starts to get really huge, I post it back up again.

Wikipedia links to unverified anecdotal evidence of the term being used at Atari sometime after 1979. According to an interview with Warren Robinett, programmer of "Adventure" for the Atari 2600 which contained an early Easter egg:

What actually happened was that by the time some kid or kids had discovered the secret room and Atari found out, I didn't work there any more. So they couldn't really punish me, and the manager of game software at that time decided little hidden surprises in games, which he called "Easter Eggs" were kind of cool. Also, it would have cost $10,000 to make a new mask for the ROM in the cartridge. So what happened was that Atari left the secret room in the game.

A student of mine said that she'd heard that the origin of the term comes from the Rocky Horror Picture Show (1975), which includes at least 3 instances of actual Easter eggs on screen. These were (supposedly) the result of an actual Easter egg hunt by the crew. Not all the eggs were found, which left several on set to appear in the film. Here's a web site post confirming this: http://www.eeggs.com/items/1914.html I have not investigated myself, and I'm not sure that this means that the term came into use because of the Rocky Horror Picture Show, but the movie predates the Atari game by four years. It's a possibility.

There are three terms: Easter egg, Easter egg hunt, and Easter egging. The third is likely the oldest, in technical use, as it was used by radio/TV repairmen likely going back to the 50s, if not earlier. I'm relatively certain I read this term in publications such as Popular Electronics in the 60s.

The meaning is the process of replacing parts "at random" (though generally with some intuition) in a radio/TV or other piece of equipment, in an attempt to find the faulty part. This was often done using two identical pieces of equipment side-by-side, swapping parts between them.

I first heard the term "Easter egg hunt" (in the technical sense) at RCA, Camden, NJ, sometime between 1972 and 1974. To anyone familiar with the TV repairman's term its meaning was reasonably obvious -- more or less randomly searching for the location of a bug or function in a mass of undifferentiated code or hardware.

The technical sense of Easter egg, meaning an intentionally hidden feature, likely represents a collision between these two older technical terms and the original literal sense of an egg hidden for a child to find, as a sort of adventure. In particular, early mentions of "Easter egg", in a technical sense, are apt to be referencing the older technical terms, especially when the inference is that the "feature" or the hiding of it is not intentional.