There is a very good chance you have a major crack and failure at some point in the wall assembly.
Can you upload a few drawings and pictures so we can get an idea of the layout of the two rooms side by side? Often shower benches are poorly build and waterproofed even worse. Fixing a few cracks in the grout will not make your shower and better but only slow the rate of waterflow.
Sadly these types of showers need a lot in the way of a rebuild. Do you have a dresser in your bedroom that perhaps could be moved and that drywall behind opened up to inspect more closely?
I find loading pictures onto Photobucket and then using the web-link that PhotoBucket generates and pasting it here on Terry's message field is the easiest way of uploading pictures.
If you fill up a few buckets of water with the shower head you can determine if the line between the mixing valve and the shower head is leaking. Fill the buckets and pour the water down the toilet.
Next you can try soaking different areas of the shower. I suspect you have a leak from the bench or right at the shower base.
Pictures are so helpful and opening up drywall on the other side really can tell a lot.