Property dunce here: brown water marks at top of wall - need help!

[TheRedRabbit]

We moved into our house 6 months ago and I noticed some very small brown stains at the top of one of walls, near the ceiling, a few weeks after we moved. I've been keeping an eye on it and it has got significantly worse over the last week, and if I touch the plaster it feels very warm, as if some CH pipes are right behind it. The room is also under a shower room and the shower has patchy grouting that needs to be fixed but I'm not sure it is a leak from here as the shower is not directly over the stain.
Had the boiler serviced recently and I pointed out the stain to the plumber, who said that if it was a leak from the CH pipes that would affect the boiler pressure, and as that was fine, it couldn't be that.
I am a total idiot when it comes to DIY/how houses innards work! What should I do? Would we need to get someone to break into the plaster where the stain is to see what's going on behind there. Who should I call? A handyman? A plumber?

I'm not an expert (although have a similar problem from an old chimney breast with faulty flashing) but remember that water flows, so the stain does not necessarily have to be right under the suspected source.

Start with the easiest and cheapest remedy (before even thinking about ripping down plaster - that'll look much worse than a brown stain!), which is to repair the grouting around the shower. Then see if it dries out, in which case, repaint.

a brown stain at the top of a wall indicates a water leak above.

If there is a bathroom or shower above assume that the tray is leaking until you prove otherwise. it will not get better, it will get worse.

Look under the floor and you may see where it is coming from. It is probably the bath/shower tray joint with the wall, but it might be a leaking waste, including the overflow.

By now the bathroom floor has probably been damaged by being wet, especially if it is chipboard which is complete rubbish. the longer you leave it the more likely you are to get rot. When you pull it up and throw it in the skip, replace with WBP ply.

Around baths and showers, use plastic sealing strip. Do not expect tiles or sealant to do the job.

Stop using the shower until it is properly fixed, and observe if it starts to dry out.

Exactly what PigletJohn says. Our was the sealant had come away from the wall and the water from the shower was running down the wall causing a damp patch in our lounge below. Simple to fix. The just used stain block in the lounge and painted over the stains.

We had a similar problem...which was caused by me trashing the sealant around the shower with over scrubbing. When I called the plumber, he said that the first thing to check would be the sealant (I confessed to the above to DH afterwards) and to give him a shout if the stains got bigger. Oddly enough, after plenty of new sealant went down, there have been no further issues.

Thanks v much everyone. I had a friend come over this afternoon and she had a look and she thinks it can't be the shower or a leak from above as the stain starts about a foot below the join with the ceiling, and it is about 4ft away from the edge of the shower tray directly above.
I'm also not convinced its the shower because of being able to feel the warm pipes behind the stain.
If it is a small leak from these pipes, would the only way to find out be to knock out the plaster?
I am going to get the shower tray sealant replaced anyway but my friend was pretty sure it couldn't have been a leak from that.

try to look under the upstairs floor first. Due to their size, water pipes cannot easily be chased into walls so are more often under the floorboards.

Water often runs along floors or ceilings and appears in a place remote from the leak, especially in some modern houses which have ceilings made of foil-backed pasterboard.