It's a fair point. One thing that we know of CB's sexual offences against against children is that they seem to take place outdoors, in public settings. The two offences as a teenager were both out in the open. The sexual offence against the 5yr old child of his ex girlfriend took place in a park when you'd think he would have had opportunity to pick a more private setting. And then there's the masturbating incident in the playground in Messines.
It's entirely feasible CB may have taken MM to somewhere nearby but quiet, and disposed of her there too. The garbage system in Luz is based on large communal industrial type bins. These are emptied between 12 and 4am nightly, each bin collected in varying frequency depending on usage. No attempts to halt refuse collections were made following the disappearance of MM so they just continued as normal. The PJ did carry out checks on bins (not sure how thorough) but the work was not completed until the 7th by which time many of the bins had already been taken to landfill. CB may well have just got very lucky unfortunately.
I also find the templates of his known offending interesting, because they fit typical profiles.
The outdoor offending is relatively typical/opportunistic. We know he committed a worse offence where he had access. Lots of electronic offences.
Then on the rape - he leaves the victim alive because there is no link between himself and the victim. This again is common with that kind of offender.
MM abduction doesn't fit his other offending but that may be just because we don't know about his other offences, or it was an escalation. But like you say, there are typically 3 crime scenes in such cases. The place of abduction. A hidden place where the assault occured (this will be a place he feels safe in/controls) and then the dumping ground - where the body is dumped of hidden.
Like you say, there is a decent chance he simply dumped the body and it hasn't been found through dumb luck. It can take decades for some bodies to be found if ever
Your idea about bins etc, a remote burial, dumped at sea etc - IMO these are far more likely than moving the victim any great distance.