Final answer:
Mr. Underwood was covertly guarding Atticus Finch and Tom Robinson with a shotgun from a window during the mob scene in 'To Kill a Mockingbird'.
Step-by-step explanation:
In To Kill a Mockingbird, during the mob scene outside the Maycomb County jail where Tom Robinson is being held, Mr. Underwood has been discreetly protecting Atticus Finch and Tom Robinson from potential harm.
Unbeknownst to Atticus and the others at the time, Mr. Underwood had been covering the scene with a shotgun from a nearby window. This reveals that Mr. Underwood, despite holding some prejudiced views, believes in justice and is willing to protect the due process of law.