It does happen, that you can spend most of your time knowing that you are getting low, and sometimes you'll get surprise readings... It does get of concern if the do start to get too frequent though.
I sleep through night time hypo's, I know when I've had one as I feel like I've got a hang over when I wake up, depending how bad the hypo I've slept through it could be a slugish feeling, to inculding headache and if really bad I wake and can't see it's like looking through really thick dark netting and pixelated... another tell tell sign is that your BG's are slightly higher than the bedtime BG...
One of the many reason of losing the hypos awareness is actually good control, where the control gained is tight and near to norm... With a non-dibetic they can get low symptons and the body will realease more sugar into the system to boost it, with diabetic the body is able to do this but it takes longer for the message to get through so you end up lower than the non-diabetic... So you don't get the symptons until you reached a much lowe point...
As SarahQ as said, some people find that raising the BG range a bit for a couple of weeks can retraing the body.. Say if your normal range is between 4.5mmol/l to 6.5mmol/l than raising them to 5.5mmol/l to 7.5mmol/l could be all whats needed... As the effect of this in the long term, well it's hard to tell and the individual would need to weigh this up...
Other reasons could be some medications such to name one blood presure tablets can mask the warnings, being more wrapped in what you are doing than you think and you've missed your normal signs even though the were there.. Different environment to name a few
I know that there was some ongoing research being carried out to see the impact of too many hypo's as one of the DNS mentioned it several years ago, what the out come was/is or who was doing the research I couldn't tell you...