In a patient admitted with a very low GCS requiring inbutation with no other reasonable clinical cause consider basilar artery thrombosis.

This can be a difficult 'call' as often even on routine scans in older patients the basilar artery can appear dense due to atherosclerotic disease.

With an in-situ basilar thrombosis, as in this case it is typically not overtly hyperdense as the clot has newly formed.  With a basilar tip thromboembolism it is classically more hyperdense and therefore noticeable on CT as the embolus has been formed for a longer period of time.

In this case less than a day later the end organ consequences are apparent as the whole posterior fossa has infarcted.