(1) The software is not doing anything nefarious
(2) The software toolchain is not modifying the software in (1) to cause it to do something nefarious
(3) The software loaded onto the machines is actually the software verified in (1) and compiled by the verified toolchain in (2)
(4) The machine doesn't have any kind of hardware/firmware-based defeat device to trick you into falsely confirming (3)
This is essentially the same problem as is outlined in Ken Thompson's Reflections on Trusting Trust [1].
[1] https://www.archive.ece.cmu.edu/~ganger/712.fall02/papers/p7...