In the Hiding Place, what was the code name that everyone used in the underground?

Mr. Smit

Peter is taken to prison after playing the anthem though he only remains there for a short time before being released. Mr. Smit is the codename for everyone involved in the underground; if no one knows the real names of those involved then they do not have to worry about lying when questioned.

faithful servants eumaeus and philaetius:

his ragged vest then drawn aside disclosed the sign conspicuous, and the scar exposed: eager they view'd, with joy they stood amazed with tearful eyes o'er all their master gazed:

these servants know ulysses by the scar he got when he was a child - so when he showed them the mark, they immediately knew this is truly their master and they hugged him, finally starting to believe his words.

he agrees with the monk's positions.