You know the funny thing is, looking at the mclaren and ferrari i would be inclined to the idea of it being a duct. It was accepted that the mac and ferrari duct put more air into the ddd. However looking on the redbull, it's hard to see how it would snake all the way to the ddd here, while being so flat.
Especially knowing that there is already a hole in the ddd to add exhaust air, why would this little one be here in front the wheel?
Maybe it's a brake caliper duct, tunneling in the floor then opening up to the face of the inside of the wheel. Placed in the wheel's silhouette where it has a redundant drag penalty. Having this here would mean the main brake duct's size could be reduced.
Another crazy idea from that duct opening to the wheel inside is an air sheet to protect the wheel or body work from excessive heat. Most turbo machinery rout some compressor air into the turbine blades to protect them.
maybe this is what RB is doing.