Lots of water in 2010 A4 rear light

[Condi]

Can anyone help, there have been 1 or 2 occasions in the last year when Ive noticed a large amount of water in the tailgate of my A4. This morning (following a lot of rain over the weekend) there was an exceptionally large amount which fell out when I opened the boot. It seemed to collect in the bottom of the boot, rather than in the lights themselves, and then comes out when you open the boot. Its not an everyday thing, so must be down to direction/rate/angle of parking or something else, but with all the LED lights and electrics I would rather seal it up somewhere.

Can anyone suggest where the water is getting in from?