"I do wonder how you land six Gliders in the middle of the night within yards of the Sentries and they did not hear anything unusual?"
While gliders do make some noise before touching down it is'nt really obvious until very close, even draggy aircraft like horsas and i would imagine there was quite a bit of AA fire going on to mask it. Go and stand at a gliding club and listen for a glider landing - until they open the airbrakes you will struggle with most types.