Barney
You had better tell us which corridor you had in mind. I will make a pretty good bet that you have either misread, or been misled by the Jeppesen VFR charts, which are well known for their inaccuracies.
The only corridor matching your general description is the one I indicated in my earlier posting, and the top for that airspace is 2700 feet ASFC.
I won't argue with you over flight planning Barney, but it seems to me that since by your own account, you chose to fly through the corridor without knowing when it was active, and without knowing its vertical extent, there is probably room for improvement.
As for the flying practises of the French military, I well remember a recent conversation I had with Rod Dean, late of the CAA GAD, in which he told me that the reason for not showing military flow arrows on British charts, was that the military will operate "anywhere" in the UK at low level, so such arrows were meaningless.
At least in France, we give you a few clues heh?