Water collecting in battery tray area

I've recently started to get a small amount of water collecting in the bottom of the battery tray compartment on my 2007 3.2 V6 quattro roadster after rain. There's no obvious reason although water seems to be collecting around the boot seal, (on the inside of the seal as well as the outside) and may be dripping down behind the trim panel. There's also a little water dripping very occasionally from the grey trim panel on the underside of the boot lid.

Has anyone come across this before? I've never had TT before (bought this one in September 2016) so I'm unfamiliar with their faults or little peculiarities!