Past perfect form of should

I have encountered a sentence ,which I’m not sure about.
This passage is about two people talking in the past. One of them is telling a story that happened before the time these two people met, so story is in past perfect.
The part I struggled is this sentence: “… Only later did they learn that corpse should have been kept for identification purposes…” I couldn’t understand why this is ‘should have been kept.’ Because it’s past perfect, I thought it should have been in past perfect. So how do we use should in past perfect?