Firstoff - Sorry for your vehicular loss. At least you are OK.
I was rear ended in my BMW on a highway and my biggest regret was NOT going to the hospital - even for a 'check out'. Sometimes bad physical shit does not happen for a bit - I have severe spinal stenosis in my lower back but symptom did not onset for a month or so - had I gone to the E/R it would have at least been on record - your adrenalin is pumping like crazy when this type of stuff happens so it's possible that you could have some minor whiplash or a neck injury. If you don't feel right go to the hospital or Dr asap - don't wait.
BTW - as far as airbag deployment - I don't believe the ass end has any impact sensors they are in the nose & the sides of the cabin so the fact that the airbags did not deploy does not surprise me.