Did the engine turn over hard while the battery was [nearly] flat? Kind of
one hard-as-nails rev per try of the key? If so, the engine can possibly kick
back a little just after getting over TDC compression on a cyl. I wonder if
that would be enough to skip the belt?
This is my guess. If you've ever tried (and failed) to start a 12V
V6 while watching the cam pulleys you'll know what I mean--when it
cranks and doesn't fire, it jerks backwards quite noticably when the
starter disengages, which is another reason why I don't trust a
tensioner on it that can't be turned backwards!