George Smiley finds pages related to 20th November from the log book missing:
Who tore out the pages of logbook, and why?
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Those pages of the logbook relate to the communications around the time that Ricki Tarr communicated that he knew someone (Irina) willing to give information about a mole. The Istanbul office was hit immediately after this, and Irina abducted.
Therefore it is likely that the mole
removed the pages, because the communications corroborate Tarr's story. The log may even show that the mole was on duty and received the communication, or was informed about the information.