Any idea of what this could be in bathroom ceiling?

Blisterman

Above our shower last week, I noticed this on the plasterboard ceiling? It was droplets. looked like condensation only it was in a ring and brown.
I wiped it off with a tissue, and the ceiling was fine.
Now it appears to have returned again. I assume its some kind of leak, but it doesn't seem to exhibit the characteristics of a leak? Why is is ring shaped and formed of droplets? We're in a block of flats with concrete floors between units.

Supercell

It is a leak, I had something like that and it got moldy eventually. In my case it was a leak caused by felt under the roof tiles being ripped, fixed that and it dried right up. There may be a pipe or a dip in the roof space and water is pooling there from a leak. Simplest thing to do is drill a small hole carefully and see does anything come through and take from there.

seamus

As above.

The flat above yours likely has an identical layout to your own, which would indicate a leak in the shower of the neighbours above you. Or a leak in the waste outlet.