They don't really do that...snakes' sperm just sort of travels on its own between the pair when they are locked. As you can see from the photo of the sperm plugs, they don't tend to secrete very much of it before it dries, and they don't extend their hemipenes unless they're breeding. The sperm plugs themselves are partially dried semen found within the hemipenal pockets. Sometimes these will be found on the cage floor or even in a water bowl--they look like the photo shown.

You're probably looking at urine or feces.