Propain spindrift missing part on bottom bracket?

Hello, unfortunately I didn't know how to phrase the question better, but I think there is a part missing from my Propain spindrift.
Picture:

If I looked at the other pictures correctly, there should still be a cover.

A few weeks ago I noticed that something was creaking at the back sometimes. But it went away and since it wasn't particularly loud I ignored it and didn't think anything of it. A few days ago the noise came back and when I did a wheelie it sounded like something at the back wasn't quite tight and it was practically rattling. But after I cleaned the rear triangle a bit with water and a brush the noise went away for a while (I thought there was dirt in some moving gap that was causing the creaking). Now it's back, quietly, and then this morning I noticed that a cap is probably missing. Well, I don't know when Couch would have lost it, but now I'm wondering if it's dangerous to keep riding like this until I find a new one (if it's even missing) and if that's what's causing the noise. I mean, the crank isn't loose at all and it's just a cap which I think is just supposed to keep dirt out.

Does anyone else have any idea where the creaking could be coming from?

As I said, everything is solid I would say and there are no abnormalities, but the loud wobbling noise also worried me quite a bit.

Thanks.