No way to know what the indent is, only your surgeon would know. Could likely tell in person but otherwise would just be guessing. May be a cut in your muscle or other?
Fat graft probably won't do much for it but no harm in trying.....is generally safe but changes are usually fairly minimal. Not all will last, usually a fraction remains and impossible to estimate on front end...to account for this, multiple injection sessions are sometimes proposed. Replacement of the implant or dramatic downsizing of the implant with addition of a "flap" can help and is usually a final/permanent fix for implant misadventures.
Hope this helps,