It doesn’t fucking exist! Period.
It’s like Wood waking up one day and going: well, I am bored, so I am just going to turn into steel today.
Muscle and fat, not only have completely different structures, but also serve different functions.
They cannot transmogrify from one thing to another. FYI I googled and found a big word to make myself look cooler.
Many a times, you might have heard stories of people that stopped working out one day and ended up converting all the muscle to fat.
Besides being impossible, what they conveniently overlook is the fact that their activity level came down, but their food intake likely remained the same.
So, no, there’s no problemo of muscle walking up to find itself in fat’s body. As long as you track your food intake, nothing bad is going to happen even if you leave the gym.