This discussion about the phone reminds me of something a senior UK detective once told me when they were investigating a money laundering case we reported to them.
This was that there is an important, but not always obvious, distinction between intelligence and evidence. Broadly, the former leads you to the perp but doesn't necessarily secure a conviction, for which you require the latter.
So someone chucks a brick through a jeweller's window, swipes a load of valuables, jumps into the getaway car, and drives off. A witness calls 999 and says that the thief drove off towards Toytown in a red 06 reg BMW. Police retrieve the brick, and police cars intercept the red BMW en route to Toytown. In it they find the stolen jewellery.
In that situation, the point about the red BMW is intelligence because it tells the rozzers which car to stop. It isn't evidence, because there are lots of red cars. The one that's interesting is the red car that has the swag inside. The case against the thief will not rely on the argument that he's guilty because he was stopped in a red BMW. It will argue that he's guilty because his dabs are on both the brick and the stolen jewellery that were found inside it. These are hard evidence he carried out the robbery and stole the jewellery. The red car just got them there.
In the same way, the phone signal tells us that someone used a phone in the area, and the phone belongs to CB (apparently). That doesn't say a lot, because he lived in PdL - but it's useful intelligence as to who to look into. And in this case, this phone that was in the right area at the right time turns out to belong to a convicted rapist, housebreaker and paedophile.
This useful bit of intelligence then sends you off looking for evidence that either convicts or eliminates that phone's owner. Or the German investigators may have found photographic or other material that links CB with MM. If they then establish that his phone was 50 miles away, that's also useful intelligence. It doesn't conclusively convict or eliminate it him, but it does tend to say he wasn't in the area.
Intelligence can accumulate into circumstantial evidence, which as noted above, can convict you all right - provided the inference from it is reasonable, and is not better explained by something that is more likely.
I suspect this, or something quite like it, may be what is going on with the curious case of CB's phone.