who'd /huːd/ is the contraction of who had (it also stands for who would). The apostrophe replaces ha in had; spelling it whod drops a required mark. The long /uː/ stays even though the written form shrinks. She asked who'd left the door open.
They found out who'd taken the last slice works because who'd can stand for who had before a past participle.
Nobody knew who'd been sitting in my chair. The apostrophe is doing its job here, and /huːd/ rhymes with food, mood and rude, all long /uː/ plus a final /d/.
Notice the sound: the /d/ at the end of who'd belongs to the contracted word itself, so nothing extra is pronounced for had.
A quick check you can trust: if who had fits naturally in the sentence, who'd is the right spelling.
The apostrophe also guards against a spelling mix-up. Without it, whod is exactly how the word whodunit, the informal word for a crime story, begins. One glance at whod can send a reader down the wrong path, so keep the mark: it shows that letters were dropped from had and keeps the contraction visually separate from the story word.
The sound gives you no clue about the meaning. /huːd/ with its long /uː/ is identical whichever expansion applies, so readers recover who had or who would from the grammar around it, not from the ear.