In my story, there is a villain who has very powerful magic on his side. Basically, he has his own dimension, where he keeps dragging parts of the earth into, piece by piece. It kinda works like the bombs in Super Smash Bros Brawl's Subspace Emissary, with the additional effect of not just concealing the location that is missing from the world, but also making everyone in the rest of the world forget about that location, along with anyone who was unfortunate enough to be at that location when it was pulled into the dimension.
The memory erasure part of the magic is needed to keep most of the world clueless, however, the people of the world should still somehow be able to find out something is wrong. One way I thought of was to make the memory erasure flawed: it removes all references of the location, but not indirect references. Suppose the villain somehow managed to swallow an entire state from the US for example, someone at some point will probably ask "Hey, why does the flag have 50 stars instead of 49?" Similarly, if someone from a football team was pulled into the dimension, someone will ask "Hey, where is player number 4? ... Wait, did we ever have a player number 4? Huh... that's weird..."
So with that setup, are there any other ways humanity might slowly figure out over time that the world is missing pieces, while circumventing the memory erasure somehow? Changes to how the memory erasure works are fine as long as it manages to keep the rest of the world in the dark for a long time.