Cat Peeing on Bed Daily?

My cat is well trained to use a human toilet, perfectly, for more than two months now, without any accident. I brought him to the vet who checked his blood 3 months ago, he was all healthy. But recently he started to pee in bed one time a day. After he pees in bed once, if he needs to use the bathroom again he would go to toilet.

I can not stay home watching him everyday, I really got stressed by that so much. The pee looks fine, not drops, should I bring him to vet? Or what reason could it be?

By Wenjing