Creating a hopper from either of the commands /fill or /setblock places the hopper but the hopper acts as a ghost block if the hopper is placed by the command in a position where it is powered by redstone. The ghost blocks do not allow the player to enter the space but instead glitches next to it, the block looks like the last block which was in the space, and no item interaction occurs. The ghost-block hoppers act like normal powered hoppers by not pulling or pushing out items and the hopper's inventory can still be interacted with with other hoppers. The hopper will output its contents if the power source is removed.

To return the invisible ghost-block hopper to a normal block, a block has to just be placed in the same position as the hopper. This can be done by the player or a command.

To replicate:

1. Use any redstone component to power a block
2. Create a hopper at the powered location using either /fill or /setblock

The ghost-block can appear as any block I tested and can be returned to a hopper by trying to view the block entity's inventory, hitting the block, or directly placing any block in the same location as the ghost-block (commands work but using something such as the second portion of a two-block block does not return the block to normal)

There are a lot of quirks. Some odd side behaviors: 

• Placing water in the same block as the ghost-block removes the water and shows returns the hopper to normal
• Item entities move through ghost-block hoppers with some resistance. If the hopper appears to be a solid block, the item entity continues to graphically teleport back to the top of the block every second or so
• Bone meal can be used to update the hopper, but only if the grass block the hopper is over bone mealed
• Placing a two tall/long block leaves inside the ghost-block half and breaks the other half. Placing the same block so part of it appears in the ghost-block replaces the ghost block with the second half of the new block instead of returning the ghost block to a hopper.
• Flowing liquids move around the ghost-block and don't update it
• The /clone command does not work to create these ghost blocks
• Using /clone on a ghost-block hopper creates the normal hopper at the new location
• Replacing a ghost-block hopper with a normal hopper fixes the ghost-block
• The debug screen shows the ghost-block as not being a hopper
• A debug stick works to interact with the hopper, returning the hopper to normal