That's not strictly true, you can format a drive to contain all zeroes. That can then be demonstrated easily via a hex editor looking at the drive. Truecrypt volumes will always look like random scrambled data (like if you formatted your drive with a "secure erase" method).
Allthough this presumes that the guy with the wrench takes the time to check out your disk in a hex editor and that he finds the statement that you directly zero everything you delete to prepare for exactly this scenario not at all suspicious. Doesn't sound like a great plan to me.
