When meshes are loaded ?

Meshes are loaded into memory when:

1. You load scene - everything referenced by it is loaded (including mesh references and prefabs with meshes on them).

2. When you call Resource.Load/AssetBundle.LoadAsset - you get reference to mesh/prefab containing mesh, that's now loaded in memory.

3. When you manually create Mesh.

4. When you're using MeshFilter.mesh (instead of MeshFilter.sharedMesh when it was used to set up mesh). It'll create a copy if it's not using an exclusive copy already.

5. When you instantiate object with its MeshFilter using .mesh instead of .sharedMesh. It duplicates Mesh.

P.S. It's also possible they're still loaded in editor from your previous non-empty scene. They will then be shown in profiler. In my case profiler's amount of Meshes loaded in empty scene dropped a lot when I restarted Unity.