About halfway through detecting poison on my diseased bard, we realized that she was in fact, diseased. We knew out of character that a Slime Mold we had fought earlier was the cause, but in-character we were clueless.
Our GM ended up allowing Detect Poison to work for examining her because he'd already started wrapping it into the plot, but we're all pretty sure that had he not bent the rules, it shouldn't have worked.
For future reference, are we correct that Detect Poison normally shouldn't be able to pick up on diseases like the one caused by a Slime Mold?