Dog pees for no apparent reason

He had just been brought inside after relieving himself. What's the problem, we were doing so well?  I've also read to put him on a schedule, but don't know what should be apprpriate between feedings. He goes to bed at 8 at night, sleeps until 5:30, eats again at noon then 6 in the evening. At each interval there is no problem with going to the bathroom outside. He doesn't ask to go to the bathroom, however, I say want to go out and sometimes he barks an afirmative sometimes he doesn't. the real problem is the interval in between 6am, noon and 6 in the evening. What do I do to get him to indicate he needs to go. I have been taking him out at 9am, and 3 pm. i've been letting him play if he goes at the between times, if he doesn't, he goes right back into the crate.