Originally Posted by cheesesteak
The telegraphed Don getting caught by Sally from a million miles away. Who didn't see that coming?
They set it up slowly.
First Sally needs to get into the apartment so the doorman give her a big ring with all of the keys.
We see her friend go out the back door to take out the trash.
Friend says she slipped the letter under the back door. ( the door Don where Don would hang out, and near the maid's room where they fooled around.
Sylvia is extremely grateful to Don for helping with the son/Viet Nam situation.
Sally wants to get the letter back so she asks for "her" key, knowing she'd get the big ring with all of the keys.
Given all the previous bits, as soon as she got the big key ring the second time it really was only going to go one way.