Google books: Ragnar Hanas hypo. There is an excellent chapter on hypos in his book
'Type 1 diabetes in children, adolescents, and young adults' , and you can read that chapter online.He states that people feel hypos in the 4-5mmol range as a result of higher average glucose levels and suggests that for a person with type 1 and good control the physical symptoms should occur at between 3.5 and 4mmol.
Indeed a normal person may not feel the physical symptoms of a hypo until 2.7. In all people diabetic or not EEG changes which detect alterations to the brain as a result of lack of glucose start at about 2.2mmol.
But it is finely balanced he also states that a person with diabetes and an HbA1c of around 5.2% may consistently not get symptoms until below the level at which the brain is affected.(2.2mmol) This can also occur with swinging levels or very frequently hypos. If you have a hypo one day you are more susceptible to another the next and this can be a downward spiral in your ability to be aware of them