The open mouth this usually happens when there is such a buildup of fluid that they can't breathe normally, so if the RI got to that point, you would have already heard wheezing.

Sometimes I hear a slight hiss or wheeze come from a snake during handling. Its nothing to worry about, but thats great that you are developing a sense of what to look for in his overall health. Some people are just completely oblivious to otherwise obvious external cues to overall health. One lady with a snake came into the pet store asking why her snake had been "shedding" for 6 months. It had probably 3 retained eyecaps and wrinkly, dry, retained shed skin on about 50% of its body. Some people, I swear!