It does apprear that the civilians depend much more on transponder information than the military.
Possibly true, but the OP question was 'Was it possible for it not to be detected?' The simple answer is if it was in radar range then yes, it was. Let's not get into atmospheric aberration, whether there was an N in the month and all of the other good stuff.
I agree with Fox, there's some serious backpedalling going on.
Edit: Sorry LL, just caught your post. ATC do show non transponding traffic, although I can only speak for certain about the UK and Western Europe. Why would they not want to show non transponding traffic? If anything they are going to be the bigger proximity threat precisely because you
don't know their height, where they are heading etc. Go flying any time in the UK and you will have ATC warning you about 'Unidentified traffic, no height information.'