There have been rumours that he may have committed up to five murders other than the five to which he has confessed (he was only convicted of three) but I believe that they are indeed only rumours, without any specific evidence. It's probably a case of people wanting to tie-up loose ends by ascribing unsolved murders and disappearances to those who have already been caught.
Ian Brady made a chilling observation - borne out by Denis Nielsen and others - if you murder a complete stranger, the chances are, you will get away with it. As the previous response indicates, rumours will always proliferate around such a high profile case, but in the end, only Brady knows, and will never tell.