Submissive urinating?????

[John Hanscomb]

We have two bitches, a 9 year old english setter/collie, and a two year old english springer spaniel. Both are spayed and both are house trained. They are not working dogs, but housepets and part of the family.The problem is with the spaniel. She is very affectionate, and loves, even demands, to sit on our laps or at our feet, particularly when we sit down in the evenings. But: when completely relaxed, not excited, often asleep, she will quietly urinate, either on the floor, or on a lap, and we cannot tell until it is too late that she has done so.
She does not have a bladder problem and she is not diabetic. She holds her urine all night without difficulty, and we often leave both dogs indoors when we go out for up to six hours, perhaps on occasion a little longer, and she is ok and dry.
We have very occasionally become cross when she does this, and she is then very upset, and clearly does not understand.
She is a sweet dog, and we do not know how to deal with this. Any suggestions will be appreciated.
John Hanscomb.