I think the only mention of a 24mp FF Pentax was in one or more of the threads arguing that 24mp 'is enough'. So not even rumor really, just wishful thinking.
I can think of only two reasons Pentax would bring out a 24mp FF:
1) To provide a lower price point. But at the price of the K-1, I'm not sure that is such a big deal. With Nikon you have D810 @$2,800, D750 @$2,000 and D610 @$1,500. Pentax gives a mix of features that are arguably better than D750 but maybe not as good as D810 at a price point not much above the D610. Plus features no one else has. Sort of "one model to rule them all"
2) To provide a faster frame rate for sport shooters. Not sure the 24mp sensor really helps there. D810 is 5fps, D750 is 6.5 and D610 is 6fps. None of those are in sport shooter range IMHO. To get there you need 1d x, or D4s in FF. Or go back to APS-C. And IMHO Pentax does not have, and is not going to have for a long time, the glass to make that kind of camera worthwhile to build.
The only thing left is the old argument, "24mp is enough". Which might be a good argument for some users, but means nothing to Pentax.
IMHO, if you want full frame, and you want Pentax it is going to be 36mp. If they do come out with a fast shooter it will be APS-C like the D500.