This has been asked on here before, so Gilby isn't unique, but I don't really know the reason.
My first Cocker did it once, when he was maybe 6 or 7 months old and fully housetrained. I had to throw the duvet out but luckily saw him do it and got the duvet off the bed before it soaked through to the mattress. He never did it again
One of my current boys (Louis of course
) once cocked his leg and had a little sprinkle, just a few drops, on a brand new duvet that I'd just got out of it's packaging to give it an airing before putting a cover on it. I think that was just him marking something new, although he's never marked indoors apart from that.
Hopefully Gilby won't do it again