Brown ring on the ceiling under the bathroom is slowly getting bigger Is This Bad?

Growing means active. Dry to the touch means intermittent, which fits a shower rather than a standing leak, and that narrows it usefully.

Before you cut anything, in this order:

• Pull the bath panel or open the access hatch and look at the trap and waste connection with a torch while somebody runs the shower for ten minutes. Most 'under the bath' leaks are the waste, not pipework in the wall.
• Check the overflow. A failed overflow gasket only leaks when water reaches that height, which is exactly the kind of intermittent that dries between uses. If your shower head is high and water runs down that wall, this is a strong candidate.
• Press the grout and sealant in the corner where the wall meets the bath edge. Soft or missing there sends water behind the tiles and out onto the ceiling.
• Dry the ceiling, draw round the ring in pencil with today's date, run the shower for fifteen minutes and look the next morning. If it grows past the pencil you have confirmed the source.

If all of that is clean, then yes, open a small square directly under the ring. A 30cm hole is a cheap repair. A ceiling that comes down on its own is not.

Dry and no smell is good news. What you do not want is soft, sagging, or musty - at that point it has been wet long enough to matter and you want somebody in sooner rather than later.

Photograph it weekly with a ruler in shot. Two months of memory is unreliable and you will want the timeline if this ever becomes an insurance conversation.

When you do cut, cut a neat square and keep the piece. Screwing it back onto a couple of battens is far easier than patching a ragged hole.

Eight months with no baths and it appeared anyway means it is not the tub filling. Something is leaking at shower volumes, which is small and regular.