Small metal part in the brake reservoir‽

Thanks!

Now after changing the brake fluid, there’s a distinct click sound from the caliper area. What may be the cause? Last season I cleaned up the caliper and re greased the slider pin and there’s plenty left of both the pads and the rotor.

There’s one initial click, which would be the brake light switch, but then there’s another coming from the caliper area after pushing the lever in further.

The lever feels firm, I think the brake fluid change went relatively well (I didn’t break anything) for being my first time. I’m not 100% confident there’s no air in the system though, I’ll keep an eye on it and I guess I’ll find out with time.

Here’s a video where you may hear the click:

It seems like something there doesn't slide well, but jumps, maybe it was like that before the brake fluid was changed, and you only noticed it now?
I would take the caliper off, clean it, apply copper grease where needed, and put everything back.
But don't take my word for granted. If you have any doubt about the brake performence, then please contact a qualified mechanic.