Epic Games Store has had a bit of a shaky start in the days following its release, though Epic seem to have successfully stabilised their product so far. Aside from random games popping up in users' accounts, that is.
Epic Games Store Kestrel is a name that people have been noticing as it appeared listed as a purchase in their account histories. It's clear that this is sort of a project name, and not an actual game titled 'Kestrel', but obviously, people are curious about just what is going on with it.
Epic Store Kestrel is, in fact, a game called Donut County, an extremely well-reviewed physics-based puzzle game where players control an ever-growing hole in the ground, swallowing various objects and whatnot. It may well be the next free release for users of the Epic Games Store, too.
Note, however, that there's no way for you to access Donut County on the Epic Store yet, short of having it pop up in your account (presumably) by accident. Additionally, the publishers of Donut County are also the same people who are publishing Ashen, Outer Wilds, and Journey on PC, all three of which are intended to be Epic Store exclusives.
In the end, since Donut County is already released on Steam, and the rest of Annapurna releases may not be in the same position, it makes sense that the publisher would try to draw users to Epic Store by offering one of their titles free of charge.