Of these mysterious tools, there is no way nobody's noticing them. I can't think of tools that would cut it that quietly. Cutting that pipe, the whole "Plumbing" which is made of steel would vibrates and would be noisy enough to raise suspicious and call for investigation... So far, nobody did, did they have more than just one help?
Didn't you know that prison guards are require to listen prisoner's breathing to ensure they are in the cell without waking them up... sounds of breathing isn't loud, but noises from cutting the pipe out of huge plumbing system fails to attract attention... 
