Brown dust suddenly appearing in bathroom Drainage and wastage systems and plumbing help, advice and answers

[NunyaBeeswax]

Hi - we’ve suddenly started getting brown dust in our bathroom. It doesn’t appear to be coming in from the window (which is mostly closed, & the dust isn’t in the room next to it with a window in the same direction & usually open). It also seems to only collect beneath items, like shampoo bottles & hand soap containers, or around their lids & occasionally in others of the floor? We’ve lived in this flat for a year & this has only just started happening. We can’t work out what it is or where it’s coming from or how to stop it? Thank you ☺️