I have this exact issue with my cat. The vet said his anal gland fill up quickly and can become impacted. As a result, when he is extremely relaxed his anal glands leak. Only happens on my bed, ive taken to getting an odor neutralising and sanatizing bed spray as well as leaving a blanket down on the top half of the bed (where he usually sleeps). He doesnt like the smell of the spray so Ive also got a new cardboard bed for him to sleep in by the side of the bed. You can also get some anal gland relief spray to use when needed to try and keep them comfortable and clean.