I totally agree with you that unless there's a specific reason for it they won't do the test, in a large part that if the hospital suspects MBP they have to be super careful about either a) tipping Susan off or b) get taken to court for 'falsely' accusing her of child abuse. That being said, though, Benadryl tends to give a false positive response for methadone. As I understand it the standard urine drug screen kits in hospitals include methadone.
Edit: This means that if he's been given Benadryl it should be able to be detected as methadone, which in an of itself would lead to further testing because, "holy shit, this kid has been fed methadone" is kind of a big deal.
https://www.ncbi.nlm.nih.gov/pubmed/20838187