It can be recessive, but more than likely the testing done in the past to determine which RH factor one of the parents had wasn't the full spectrum test. There are Two RH genes, RHD and RHCE and general testing doesn't include both spectrums. There are also other genes that can cause confusion in RH factor testing, Duffy gene and a few others. - there is also the possibility that the test done on the babe wasn't sensitive enough to read his positive RH factor also

it's not as cut and dry as a paper blood clot test, nor do most labs test conclusively - if the test results lean one way they go with it, which can end up with inaccurate results

its not unheard of a woman KNOWING she is positive and being tested later and finding out she's negative and vice versa.