I see the same thing over and over again. I haven't find anything about it so far.

[edit] After some forum searching, it's apparently related to "Mesh size" under Map Properties not being 0. From what I found, it causes no blocks to be around your level (because they're meshes instead?). If you set it to 0, the warning goes away... but so did my skybox O_o

I'm not sure what the proper way to proceed is in A7. [/edit]