Inattention is often characterised as excessive theta wave production. When certain parts of your brain is in a theta wave state its the equivalent of your thoughts in a random state. What percent if any is truly random who knows.
When you dream or daydream your mind enters that theta state. I've noticed right before I fall asleep sometimes. My mind goes wildly random. Which may indicates the theta waves are peaking.
Since inattention is quite common on the autism spectrum , and ADHD and a few other conditions have alot of "random" thoughts its
a safe estimate that around 10% of the population has these thoughts. It amount to a pretty good definition of a neurological basis for above average creativity to.