Minecraft wiki entry for Redstone_Comparator, associated text: When a comparator measures a large chest or large trapped chest, it measures the entire large chest (54 slots), not just the half directly behind the comparator. A chest or trapped chest that cannot be opened (either because it has an opaque block, ocelot, or cat above it) always produces an output of 0 no matter how many items are in the container — shulker boxes can always be measured, even if they cannot open.