Please help: There is a corner shower which we think may be leaking

That is water stains, water has found the lowest points, could be the joints of the ceiling plasterboards.

Its definitely a water stain. You'll have a joint in the plasterboard there so as the water gets into it, it tracks down the joint forming the staining you see in your photo.

You do have an ongoing problem. If it is not clear water then it is foul water from a poorly sealed shower ..so if it is ongoing its mains water you would really know about it.
If it happens after using ur shower then its the waste water. Solution ..
hmm. Check waste water pipes for leaks, check the seals around the tiled shower tray for leaks....this means getting access to under the shower tray . You could buy loads of
silly con as a temporary fix .

Its a very common problem with showers sited on wooden floors and which are made of poor gauge plastic so they move around. And also poor sealing between the tray and the finished tiled wall .

centralheatking

If your shower valve is a concealed unit, take the faceplate off and check the water connections for and leaks. You'll may also need to access the pipework and check any joints for leaks.

You will need to gain access under the shower if it is not leaking at the valve or associated pipework. When you can see underneath the tray, have a look at the pipework and see if there is water on any of it or running down any pipes. Run the shower and take a look. Be sure to spray the shower over any walls within the enclosure with water and take a look underneath. If it isn't pipework feeding the shower leaking, it might be the waste pipe so again, inspect after water has been put through it. It could be the seal around the tray or through the grout or panels or whatever method was used to make the enclosure.

Post some pictures of the area if you are struggling to uncover pipework or gain access.

9 times out of 10 its usually a poor silicone seal between the tray and wall , just have a good look if you can push a credit card through the seal that’s probably the problem.

Is it a tiled bathroom? What kind of setup is it?

If the shower hasn't been tanked, doesn't matter how much silicone you have around the place, it's going to leak if it isn't waterproofed.

Either tile backer boards, or paint-on glue. But without it, your water draws into the grout joints and adhesive which are both porous. And then eventually ends up coming down the wall to where it isn't tiled under the bath/shower, and does that. Plasterboard ceiling will eventually fail. And it's a huge re-tile re-roof jobby.

Joannec

Is the stain wet and or growing? Or is it a one off from an exuberant showerer?

If it is ongoing, you need to get to the root of the problem / source and that is unlikely to be a quick / easy fix.

Whatever, don’t leave it, unless it is the former ( a one off) the situation will deteriorate rapidly. If in doubt ( and if possible) restrict use if the shower until the problem is resolved.

With respect to the ceiling. Once plaster / gypsum based plaster board has been soaked it should be replaced - it never properly recovers, even after drying out, it always retains the affinity to attract water.

A bit like me and red wine!