I have posted on this topic before but have now identified where water is leaking in. Three images are of the right hand corner of underfloor cavity. Water is coming from behind sealant bead on vertical edge. Tonight after about 1 hr of light rain water has accumulated and can be seen in photo. 

I visited my dealer yesterday but they had no answer as to the source suggesting I keep investigating.

I want to know what is behind this panel? Looking underneath bumper I cannot see any thing that might be a cavity nor workout where water can come from to penetrate the sealant.

I could add more sealant but clearly that doesn't address the core issue. 

Is there anyone who can give me an insight into the body structure on the offside rear please.

 

I posted this on the other thread…

AjhflyerWe had huge water ingress issues on our MKI Focus, with water gathering in the wheel well and the underlay was all wet on the os of the car and water in the footwells. 
We had the car in a main dealer about 5 years ago, who stripped the scuttle, plenum chamber and removed the OS wheel arch liner; they couldn’t find anything and I was £450 lighter. Time went on, with the car soaking in the winter and dryish in the summer. In November last year I took it to a small local independent who found the issue in 10 minutes. The seam joint where the boot cross member joins the lower scuttle had split in two places. Water running down the body and behind the bumper cover, which hides this joint of course, entered these gaps, soaking the boot carpets which, in turn, then dripped into the wheel well. This now soaking carpet, which goes behind the rear seats, soaked the interior carpets but, as they’re waterproofed, the underlay became saturated but the carpets remained dry; there was an inch of water in the rear and front footwells. Anyway, they dried the carpets and underlay, sealed the joints and voila - no more water. Worth a check. 

Above advice is logical. I have looked at schematic of rear bumper assembly to identify the various components. Water rounds down side of tailgate and then drops behind bumper. Below bumper is the cross member located behind the crash bar. I cannot see any water flow when I am under car but it again seeped through sealant on inside as per my photos. I cannot reach outside area to check for poor sealant so have applied a silicone bead inside and will monitor. If sealant has failed on outside I wonder why but I cannot see a big risk in only resealing the inside.

Thanks for input.

are you sure the water is not leaking through the tail light and running down the sealant to the boot floor?

14 hours ago, UndertheRadar said:

are you sure the water is not leaking through the tail light and running down the sealant to the boot floor?

As above, lamp clusters the most obvious area, very unlikely, but not impossible, that a seam is leaking. Try track back from the leak with water soluble marker, making marks as you go, the water will erase the marker. 

I had a similar water leak on my  old fabia estate turned out to be leaking through the rivnuts  that were used to hold the bumper support in place, it couldn't be seen for the inside as it was in the Box section, might be worth a look