The slab is carved in relief. Pass the lamp over it: something only reflects light at a raking angle.
The pan needs exactly twenty-four ounces. Not one more.
Four knocks on the plate. Play them back with the same spacing.
Four candles, burned down unevenly. From tallest to shortest: swap two to fix it.
The slab gave way, and underneath is an oak chest with four locks and a frieze carved on the lid. Each lock yields a medallion, and none of the four opens anything on its own: what opens the chest is the order the medallions go in. That order has been carved in plain sight for three hundred years, on the lid, and meant nothing until now.
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