It's not a silly question. There is no evidence they didn't investigate.
There is substantial evidence they did investigate the phone evidence. They just didn't investigate it thoroughly enough to note the calls made by those who appear to have been petty burglars, who were in communication and who were in PDL at the appropriate time.
That the evidence was there is indisputable ... the PJ found it as did Operation Grange ... seven years down the line.
They concentrated everything on Madeleine's parents and Murat and cast the net no further.
Robert Murat and his associates were thoroughly investigated ... as an arguido that was perfectly in order.
Madeleine's parents and their friends were also scrutinised ... as the people closest to the missing child ... again a proper investigative strategy.
That a measure of cross checking was carried out is apparent from Inspector Paulo Diaz' report ...
a) In the first phase, we proceeded to a careful reading of the files taking notes of the elements that could contribute, in any way, to the composition of the chronograms of the facts. From that reading is clear that much of the proof is testimonial.
b) So, at this phase, we compared the declarations of the Ocean Club's employees with the phone registries of the 3 operators, to figure out if there is any incongruence between the depositions made and their presence at the place, when they claimed that they were absent from that locality.http://madeleinemccannfiles.blogspot.co.uk/2012/10/PJ-1a-1s-Intelligence-Analysis-Report-05-02-08-PJ-Inspector-Paulo-Dias_29.html