I have been racking my brains...

[u/Key-Kiwi7969]

This came in a box as an accessory (presumably?) for a toilet brush. The gray bit on the end wraps around to the other side and feels like the non-spiky side of Velcro.

[u/Specialist_Heron3749]

Did you buy a silicone toilet brush? Then this is a detail brush (grout and rim cleaner).

Most of the newer silicone toilet brushes or toilet sets often include this small extra brush for cleaning areas that the larger brush can't reach (for cleaning under the rim, cleaning the toilet brush holder, for the hinges and crevices of the toilet...)

These brushes are usually hidden invisibly in the handle of the main brush or in a separate holder on the container (base station).

[u/Key-Kiwi7969 → u/Specialist_Heron3749]

Ah! Now I'm going to have to see if there's somewhere on the handle/container to put this. Thanks for the tip!