Had the same issue, it was basically, the hatch where the lock clicks into was loose and moving around because the bolts holding it in weren't secure, that's located on the wall of your door, looks like a nose ring sort of thing with black padding around it to hold the lock in. If that's worn or faded the lock might not be engaging properly, hence the light is coming on. Also you'll notice that when you close the door, the reason the light comes on and dissapears in the normal case is because it forces a button down, might be an idea to check if that's working properly too by pressing it down manually and looking at your dash. This stuff is all relatively close to each other on the door arm of your car's body ( open your door and look to the left im talking about the part that runs up from where you can rest your feet on the side of your car)
If it's a matter of the black padded material on the metal ring/arch that hold the door locked in place being faded or worn, just wrap some black electric tape around it until it fits snug again or find a replacement for it..
If that isn't the case maybe it's something electrical.
And Don't directly use water on your engine bay.. E.g dont spray it with a hose lol.. hope you didn't. Just a damp cloth or preferably de-greaser or something.