Why does my male dog urinate on beds?

I have 2 Japanese Chin dogs. They are siblings and 11 months old. They are both housebroke and the male is neutered but he occasionally will urinate on beds if he is able to get to one. It doesn't matter if the bed is made neatly or if the sheets are clean or used a few days. The dog isn't particular. Everyone in the family has their own ideas as to why he behaves like this. He and his sister are both lovely pets and the breed is everything you could wish for in a fun family pet except for this one quirk. Anyone else ever had this experience?