During the firebombing of Dresden, Oskar's grandfather burned his hand on a metal doorknob, all that was left of his home. The image becomes "burned" into his mind, and before he left New York, he took photos of all the doorknobs in their apartment. In some of the photos the doorknobs are locked, in others unlocked. (See: "Should He Stay or Should He Go?" in Grandpa's "Character Analysis." Clearly, he's not sure what he should do.) Opening doors can have a positive connotation—exploring new possibilities. A locked doorknob could mean thwarted possibilities. And a doorknob on fire would make you completely helpless.
And doors have keys, of course, so this imagery mirrors Oskar's journey as well. Would Oskar's "key" have helped Grandpa if he'd been able to stay around?