Water Leak

Hi all.

Just wondering if anyone knows what's causing a long standing leak on my old boulevard, I never see drips etc but the carpet is usually wet after rain, and it was the only MOT failure in 8 years when a small plate had to be welded over a hole made by rusting on the passenger side. M y mechanic thinks it may be down to a leak from the front or windscreen draining system? Any advice please? DC