If you have; BP_Foo, that has a ChildActorComponent of BP_Bar, and you set the Mobility of BP_Bar to Moveable, then place BP_Foo in a level and cook it. BP_Bar will end up at origin with a static attach warning.
If you however leave BP_Bar as static inside of BP_Foo, then place BP_Foo in a level, then set the Mobility of BP_Bar ChildActorComponent to Moveable, that will cook correctly with no attach warning.
See description/UDN
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-215536 in the post.