"Mrs.Tynn protests that no one could have approached the desk unseen by her.

It is very unlikely that any one could have burnt it.

They must, first of all, have chosen a moment when my uncle was asleep; they must have got Mrs.
Tynn from the room; they must have searched for and found the keys; they must have unlocked the desk, taken the codicil, relocked the desk, and replaced the keys.

All this could not be done without time, and familiarity with facts.

Not a servant in the house--save the Tynns--knew the codicil was there, and they did not know its purport.