Deductive reasoning. Police haven't shared a whole lot of information about the case, but they have been pretty open about their trouble pinning down when Zahra was last known to have been alive. LEO has spoken to the mother directly (by phone) and I expect to the grandmother as well, and would certainly have gotten access to phone records by now via Australian authorities assisting. It's only in the past couple of days that they've come up with a "sighting" that puts her alive any more recently than about 6 weeks before her disappearance. Grandmother has also spoken to the press and hasn't mentioned anything about when she last spoke to Zahra. So not a peep about any phone contact with her since at least the time of the last change of residence.
Also, if grandmum (and/or mum) had been speaking to Zahra periodically by phone, it's hard to imagine they wouldn't have detected at least a hint of trouble in her tone of voice, even if she was being monitored during calls by EB/AB and not free to openly make any complaints. And if I were a loving grandmother or mother, whose young cancer survivor, amputee grand/daughter had been half way around the globe for two years with her not-too-bright dad and his "gothic fairy" internet bride, and I thought I heard even the slightest hint of worry or unhappiness in her tone, I'd step up the phone call frequency and notify Australian child welfare authorities if I found I could no longer reach her. I'd also be insisting, either directly, or if blocked by EB/AB, then through Australian child welfare authorities, on confirmation that the child was receiving appropriate follow-up care and monitoring for a cancer survivor and amputee.
