In the Enchanters’ End Game, the last book of the Belgariad series, the following exchange between Belgarath the sorcerer and Silk takes place, when they discuss how the Sword has been hidden:
“It’s still there,” Silk noted, sounding a bit disappointed. “I can still see the sword.”
“That’s because you know it’s there,” Belgarath told him. “Other people will overlook it.”
“How can you overlook something that big?”, Silk objected.
“It’s very complicated,” Belgarath replied. “The Orb is simply going to encourage people not to see it - or the sword. If they look very closely, they might realise Garion is carrying something on his back, but they won’t be curious enough to find out what it is”.
To me, this sounds a lot like the description of Somebody Else’s Problem:
The Somebody Else's Problem field... relies on people's natural predisposition not to see anything they don't want to, weren't expecting, or can't explain. If Effrafax had painted the mountain pink and erected a cheap and simple Somebody Else’s Problem field on it, then people would have walked past the mountain, round it, even over it, and simply never have noticed that the thing was there.
The German role-playing game The Dark Eye also has a similar spell, called Harmless Shape:
You take the shape of an inconspicuous person (such as a servant or beggar) that does not attract attention at the current location. The illusion changes your appearance and voice, but does not grant knowledge of things like languages or appropriate behavior. This spell does not hide larger objects or familiars. The spell itself picks the shape automatically—the caster cannot choose which appearance to take.
As the comments have pointed out, Terry Pratchett used a similar idea, and TVTropes lists something along the lines as well.
I was wondering, when and where was this idea first mentioned?