Talisman
John keeps Harold's cufflink.
He doesn't even realize he's done it at first.
He dozes in the library with Finch and Bear until Finch starts making pointed remarks about him sleeping better in his apartment (which they both know isn't true).
Restless, he doesn't make it back to his apartment until the sun is rising. When he goes to change his pants, he's surprised to find the cufflink in his pocket (takes a moment to look at it like he's never seen one before).
He transfers the cufflink to a clean pair of trousers and makes a mental note to return it to Finch.
Six numbers and quite a few bandages later, he reaches into his pocket and finds it there again (closes his fist around it and listens to Finch type reassuringly at the computer).
When you're in trouble, you can call DW.