It happened twice at Sierra, around the same time in fact. I have found part of the AGI interpreter source code on a King's Quest III disk (version 2.14, 720KB version, disk 1). There is nowhere near as much, but lucky for us, it happens to include a few complete files that were not on the SQ2 disk. So that increases the total amount of source a bit.

I have checked other Sierra disks, and so far the SQ2 and KQ3 occurrences are the only ones I know of.