This shit is just a Bermuda Triangle of balls.
What do I mean vanish? I mean take the fuck off. Pack up their bindle stick and head out for a life on the road. And it's only ever one at a time, as though your nuts had a heated argument and one forced the other to go spend the night at its parents' house. So your sack will still be slouching there against your thigh like a drunk trying to stay on the dance floor, and one ball will be holed up in there like the Unabomber in his shack. The other one will be three fingers deep in your torso, exploring the nooks and crannies of whatever the fuck is equidistant between your dick and your butthole.
To the best of my knowledge, your balls will occasionally rise up like the disgruntled citizens of an unjust regime, but it's generally for warmth. This isn't that. This is like a lava lamp situation, with blorpy stuff just oozing about because it can, no real rhyme or reason behind it. The little vagrant will always return home in short order, no worse for the wear and tight lipped about what sights he may have seen. But know that sometimes, when you least expect it, there's a nut that's just gone walkabout like a little semen-producing Crocodile Dundee.
"I have a journey to go on, my friends. You won't understand it, but you will be in my ball heart." - Your nut sack