World Bowser-1: Spiky Spike Bridge
At the ledge near the first Warp Box, you can see an 8-bit Luigi on the side of the ledge.
World Bowser-2: Plessie’s Dune Downhill
Once you’re done with your ride with Plessie, you’ll find an 8-bit Luigi imprinted on the grass beside a tree.
World Bowser-3: Cookie Cogworks
Find the binoculars near the Checkpoint and use them to look toward the wall on the left to spot another 8-bit Luigi.
World Bowser: Train the Bowser Express
During the intro cutscene, you can see 8-bit Luigi outpacing the train above. Later in the level, right before the Warp Pipe, you can see Luigi on the side of the train. Maybe he got tired?
World Bowser-4: Footlight Lane
On the back of the first Thwomp of the level.
World Bowser-5: Deepwater Dungeon
Shortly after riding Plessie, climb the right wall in your Cat Suit. Then, jump to the adjacent wall and walk along the top of it until they reach the third stone block, where an 8-bit Luigi is hiding.
World Bowser-6: A Beam in the Dark
If you wait at the first mirror until there are only 90 seconds left on the clock, a large 8-bit Luigi will pop out and phase through the level. And if you make it to the flagpole with 30 seconds, you’ll spot him again on the moon.
World Bowser-7: Grumblump Inferno
An 8-bit Luigi can be seen on the second Grumblump.
World Bowser-Boss: The Great Tower of Bowser Land
At the start of the stage, hit Bowser’s car, and an 8-bit Luigi will fly out.
There are two hidden 8-bit Luigi’s on this map. The first is riding the Ferris Wheel, and the second is on the wall near the Toad House.