LUKS can be easily detected.
The specifications for the on-disk format are published online.
http://code.google.com/p/cryptsetup/wiki/Specification.

What I would recommend and personally employ... First, fill the disk with a random background:
# cryptsetup --cipher=aes-xts-essiv:sha256 -s 256 --key-file=/dev/random create mapper1 /dev/sdz
# cryptsetup --cipher=twofish-xts-essiv:sha256 -s 256 --key-file=/dev/random create mapper2 /dev/mapper/mapper1
# dd if=/dev/zero of=/dev/mapper/mapper2 bs=512

Don't bother creating a partition table or anything else. Leave the entire disk full of this background data.
Then create an encrypted volume using a hash for key material and offset and skip sector counts from the hash string:
# echo "secret_password@drive_serial_number" | sha512sum
4839 eeac 06 a 2045 d 60 6dbf519ba5e9[...]e312009896441a5
# cryptsetup --cipher=twofish-xts-essiv:sha256 -s 256 -o 483906 -p 204560 create encrypted /dev/sdz
Password:
# pvcreate /dev/mapper/encrypted
# vgcreate
# lvcreate

If questioned I would respond with nothing, no words, and just chill there.

For all of you /.'ers out there there's an interesting new technology out there to detect these types of flaws. I'm a nuclear student at UF and some in our department are working on lateral migration radiography. It's a rather cool process, shoot x-rays into the foam and get an image of what's inside and find out where delimanation or debonding has occured. http://www.nre.ufl.edu/facilities/backscat.php