She might have needed her diary ASAP to get numbers she needed to call that day - for social reasons maybe but also business contacts perhaps if she was involved in a deal. Though unclear if she could make personal calls from her work landline.
The importance of the diary - the equivalent to a smartphone really - to her life is why I think if she'd lost it on the Friday, the people she was around all that weekend would have heard about it. Did she really not call anyone that weekend? Surely she'd have mentioned it to her mum. She'd have been anxious about it. Wondering where she left it. Calling Mossops. The pub if they really went there which AL later denied. Calling AL asking if he'd picked it up. He'd have known about it surely as she was with him on the Friday.