By the time they returned to the apartment, they found it stripped of most of the personal belongings, the few pieces of furniture left behind remaining as the sole witness to what happened.

A call to the 9th precinct on their way over had revealed that O'Donnell never showed up for work today, not answering his cell, the latter having been tracked to a dumpster six blocks down the road. His work history was nondescript; a few commendations for bravery summing up a two-decade long career in law enforcement.

No wife, no kids, no siblings and no parents left alive would make it difficult to find his lair, so Beckett had spent half an hour on the phone with his Captain, trying to see who O'Donnell's friends were or if he had any other contacts outside the department that would be worth investigating.

In the meantime, they'd looked into the robbery detective's bank account, finding it completely emptied as of early morning. Several large withdrawals had been made over the course of the past weeks, coinciding with the murders and subsequently draining his 401k.

It wasn't hard to speculate that O'Donnell was on the move.

Whether that meant he was on the run from law enforcement or merely going into hiding would be something they'd find out soon enough.

They'd turned the barren apartment upside down for the past few minutes and yet Castle felt the urge to keep looking, practically sensing that there was something left behind, something that would help them close in on their killer.

While Ryan and Esposito made phone calls and Beckett inspected the bedroom, he took a moment to look around the kitchen and dining room, the scraped-up floors a sign that the area was well used by its occupant.

A cheap pine table with two chairs tucked against the outside wall seemed to be the masterpiece. The kitchen itself was standard fare, the oven not having seen cleaning in a long time. According to the microwave, O'Donnell had had mac and cheese the night prior.

As he shuffled his way back to the drafty window frame facing the road below, ready to check the brick wall for any crevices hiding secret messages, Castle felt the floor give just a little.

Stopping in midmotion, he glanced down, curiously studying the red rug beneath, then carefully backtracked his steps, this time feeling a distinct change in the floor panels.

"Guys, I think I got something!", he announced triumphantly and pulled the rug off to the side, coming face to face with a small compartment cut into the floorboards near the kitchen counter, cunningly hidden out of sight.

As rapid footsteps approached his position, Castle reached for the small handle screwed intro one of the boards and pulled it open, immediately taking in the aroma of old paper.

"What is it?", Beckett asked and crouched down to join him while the boys stayed to either side.

"Looks like some magazines maybe? And flyers?"

With his gloved hand, he reached inside the box, immediately making out the German Fraktur typeface and loathing what it would say.

"Reichsjugend Magazin…", he read out loud, proud of his proper pronunciation, then held up the small literature that contained only a dozen pages, "Looks like World War 2 propaganda…except it's written in English. And O'Donnell is the author."