Zambrano gave up a career-high five home runs earlier in the night and was ejected with one out in the fifth inning after pitching inside on consecutive pitches to Braves third baseman Chipper Jones.
He later packed up his belongings and left without speaking to Quade or other players. Several Cubs teammates were unaware of the news until being informed by reporters.
"That means he's had enough, then," outfielder Marlon Byrd said. "People retire for a certain reason and they're done with the game. If that's the case, I wish him well. I don't know. I have no clue.
"He's talked about it before when I wasn't here. I guess we'll see tomorrow. If he doesn't show up, then we might not see him again."
Byrd didn't want to speculate on why Zambrano may be deciding to retire, saying that he would try to reach out to the right-hander when he left the clubhouse.
"There's a lot of things that happen in people's lives that we don't know about," he said. "A lot of things could be happening at home. We don't know until we talk to him. ... Hopefully, the rumors about him retiring aren't true, and he'll be back tomorrow and we can talk about it."
Quade is upset, but not because Zambrano didn't inform him of his decision. He's upset because Zambrano left his teammates behind.
"He walked out on 24 guys that are battling their [butts] off," Quade said. "I don't know where he's gone or what he's doing. I heard he might retire, or he's talking about retiring. I can't have a guy walking out on 24 guys. That's for [darn] sure."
Byrd has seen a teammate walk out before, but it's a first-time experience for third baseman Aramis Ramirez.
"I've never seen somebody just grab their stuff and leave and retire," Ramirez said. "And I've been around for a while. Players don't do that. He's been playing for a while, too. He knows anybody can have a bad game, a bad week, a bad month. It happens to everybody. He's not the only one."
Ramirez believes that Zambrano -- who earlier in the year called the Cubs a "Triple-A team" and "embarrassing" -- would be welcomed back.
"I think so. I can only speak for myself," he said. "I've been playing with him for a long time. I know he's not a bad guy. He just doesn't know how to control his emotions sometimes."
Byrd understands that Zambrano can be emotional.
"He's a sensitive guy," Byrd said. "Maybe tonight was just too much for him."
Whether or not Zambrano's decision is official remains to be seen, but Byrd isn't ready to throw in the towel just yet. Zambrano has one year remaining on his five-year, $91.5 million contract, with a vesting option for 2013.
"I see him being frustrated and thinking about himself right now," Byrd said. "If he goes home for good, then it's a different story. If he shows up tomorrow, then we'll see what's going on."