When does Auden feel that suffering usually takes place

when people are doing ordinary things, such as eating or walking

Suffering usually takes place when the people least expect it.

"About the suffering they were never wrong, the old masters: how well they understood its human position; how it takes place while someone else is eating or opening a window or just walking dully along."

In the excerpt of the poem Musee des Beaux Arts, W. H. Auden expresses that suffering happens while others are just going about their usual, normal day-apathetic and not expecting it to happen. The poem painted the people indifferent to the suffering of others and just continued on with their daily routine.