This is no doubt an unjustified leap, but I wonder if a miniprotein could be small enough to be a candidate for the the proverbial "something in the blood" ?
Mice can leap a long way... The smallest of these microproteins are around ten amino-acids long—the average for proteins being around 300—so I imagine that they're small enough. That Science article doesn't go into the specifics of the recent Salk paper in Nature but the findings are even more interesting. The 54-amino-acid, PIGBOS microprotein that they've characterized is used to communicate with the mitochondria to help it with a stress response. Indeed, these small proteins seem to play helper roles in a variety of contexts so some might well circulate in the blood and might well have been overlooked. For more on this fascinating field, see this Salk article: https://phys.org/news/2019-10-mysterious-microproteins-major-implications-human.html And see also this Salk article on another microprotein, one called NoBody: https://phys.org/news/2016-12-microprotein-mission.html