I agree that it looks like it crawled out of that tower to the right of that doorway. There’s gotta be a block there that’s at or below the light level needed for mobs to spawn. Or else they could have wandered in from outside the base after spawning in the shade of a tree? If you use bottom slabs to cover the flooring, mobs can’t spawn. Then you could see if they’re spawning somewhere else and then entering your base
mobs spawn at light level 0 now and there’s no chance. i shifted the basement hanging lights to every 4 blocks instead of every 6 even though nothing has spawned down there before. there’s no heavily shaded areas outside the base and there are covered sea lanterns every 6 blocks on the ground and it’s very well lit. same for the compound the base is outside of and nothing has spawned there for generations.