Can an implant move from under the muscle to over the muscle on its own without surgery?

The implants do not move from under to over the muscle on their own or with massage.  There needs to be a space created surgically to allow for that.  So it may happen if you had an implant over the muscle and then switched to another implant under the muscle.  If the old pocket was not closed, the new implant can slip out of the new pocket and end up in the old pocket over the muscle. 

Breast implants are often placed under the muscle but sometimes a different pocket is used, such in the case of a dual plane breast augmentation.  In any case, the pocket that is used is a usually a defined space, and the implant itself could not move into a different space without further surgery.  Patients who have had trauma to their breasts or have massaged them often may have this concern, but in reality it is not something to be worried about