Sam is the one who suggests the Hide-And-Seek Initiative, as it came to be called (Clint). Bucky has good days and bad, but he hides on the bad days and it usually takes the entire team to find him. He never leaves the Tower, but it's...nerve-wracking. Sam wants them all to start hiding; one Avenger starts the finding process, and then each located Avenger helps the first...and it snowballs pretty quickly from there. Steve just thinks it's unfair that more often than not, he turns around after someone announces the game to find that everyone else has vanished, and he winds up "It" by default.
Bucky is always last to be found. Despite Nat's skills and Clint's penchant for hiding in duct-work, Bucky is the one who drives them all mad as they try to figure out where the hell he's gone.
Surprisingly enough, it's not long before everyone realizes that Tony, of all people, is second-best at hide-and-seek.
Steve has a moment of thinking about child-sized Tony learning to hide from Howard, and needs to be alone for a while. Tony doesn't want pity and he gets baffled by sympathy, and Howard is too many years dead to punch in the face.
It gets to the point where Bucky and Tony one-up each other on hiding spaces so much that they both manage to hide for twelve hours. JARVIS finally takes pity on the seeking Avengers (plus Pepper) and reports that Tony and Bucky gave up on being found six hours ago, and had re-hidden themselves in the workshop behind a hologram showing an empty room.
Once the missing have been found, Clint swears until he starts turning purple and insists that he checked the workroom eighteen different times. Bucky looks up at the air duct opening in question and says nothing. Tony just grins.
Either way, the metal detector trick is how they finally start finding both of them at the same time. It begins with the couch. Bucky starts swearing; Tony pops up from a previously unknown hatch in the middle of the floor: “He's where? Jesus, Barnes, you’d better have not sliced up my fucking couch. That is leather.”
"What kind of leather?" Steve asks, curious. The couch doesn't look like leather. He thought it was one of those new synthetics.
Tony pauses. “Leather from…uhm…not-cow? I don’t know these things. I pay other people to tell me that I’m buying expensive not-cow leather. For all I know, it’s made of mongeese—BARNES, DON’T YOU SLICE YOUR WAY OUT OF THAT COUCH!” Tony yells, and Steve looks and winces because Bucky is doing exactly that.
"Got stuck," he says. "Mongeese?"
"Isn’t that the plural of mongoose? One mongoose, two mongeese?"
Then Bucky hides behind one of Pepper’s modern art canvases that hangs flat against the wall in the penthouse, the one that's ten feet tall and eight feet wide. Steve stops in front of the painting, metal detector in hand, incredulous. "Buck?"
Then Tony says, “What the fuck?” and Steve drops the metal detector on his feet, because Tony literally appears out of nowhere. There is a pattern of color over him from head to toe that exactly matches the wall he was standing against.
"Did you invent a holographic camouflage suit just to win at hide-and-seek?" Steve asks in disbelief, picking up the metal detector.
"And I would have won, too," Tony grumbles, as they get on each side of the painting and take it down. Bucky is half-crouched in a hole in the wall, one with rough edges that's about four feet high and three feet deep.
"Bucky, this may be getting out of hand," Steve begins to say, but Bucky rolls his eyes.
"I wouldn’a had to resort to this if you weren’t cheating all the damn time."
Steve draws himself fully upright, indignant. “I am not cheating. We find everyone else first, and we keep searching for you guys for another four hours before I break out the heavy artillery.”
Tony stares into the carved-out space once Bucky finally deigns to come out. “That’s a steel-reinforced wall.”
Bucky looks nonchalant. “Yeah.”
"Did you seriously spend hours punching a hole in my wall with your metal arm just for a hiding spot?"
"Yeah?" Bucky says again, tilting his head.
"That is awesome,” Tony says, which is when Steve gives up.