bathroom leak and damp carpet on landing advice please

[ROUNDandROUNDINCIRCILESMORETHA]

Hi
lately i have noticed a damp smell on the landing thankfully as its not been redecorated yet i pulled away the paper to find the wall was slightly damp from the shower
as time gone on i noticed smell getting worse and decided to do more investigation. Pulled away mirror to find now carpet is also damp at the bottom of that wall.
I hope that makes sense what advice i need is is it because sealant was applied poorly OR could it be a more serious problem as in a leak in the bath somewhere. I cant pull away side of the bath as the guy who did our bathroom tilled the bath side against our wishes. I wish now i had gone with a professional company instead of supporting a friend who had just started out :-(