With TrueCrypt you can create a hidden partition and place a full OS in there that is "undetectable"
A hidden operating system is a system (for example, Windows 7 or Windows XP) that is installed in a hidden TrueCrypt volume. It should be impossible to prove that a hidden TrueCrypt volume exists (provided that certain guidelines are followed; for more information, see the section Hidden Volume) and, therefore, it should be impossible to prove that a hidden operating system exists.
are there any FOSS tools out there built into Linux that would allow this without the use of TrueCrypt? Not that I dont trust TrueCrypt, on the contrary I do, just wondering if there are other/better solutions out there.