The venomous mangrove snake that escaped its enclosure at the Bronx Zoo was still on the loose Thursday — as officials said they believe it got out via a vent.

The Post revealed Wednesday that the 3-foot, tree-climbing snake went missing from its glass case the night before — but the zoo kept the area open, erecting one little sign warning guests.

Zoo officials ignored repeated requests to explain what happened, but told CBS2 they believe the fanged fugitive somehow got out through a mesh-covered vent in its enclosure and is still somewhere in its Jungle World exhibit.

A sign on Jungle World notifying visitors of the missing serpent — but downplaying the danger — was still in place Thursday, with workers apparently failing to locate the snake after searching all day Wednesday and into the night.

But the small, letter-sized sign at the counter wasn’t noticed by most people lining up to enter the exhibit — and many weren’t happy to hear the news.

“I would not have stepped a foot in this freaking zoo with my niece if I knew there was a snake missing,” said Breyanna Wright, 21, of Brooklyn.

“Who the hell says, ‘There is a 3-and-a-half foot snake missing, if you see it let us know?’ Hell no. That’s not cool,” she fumed.

There have been no known mangrove snake fatalities, although their venom can cause painful swelling, according to the Smithsonian’s National Zoo & Conservation Biology Institute.

But the sign claimed there was little chance people would encounter the missing mangrove.

“Dear visitors, a 3 and a half foot long mangrove snake is missing from its exhibit in Jungle World. They are mildly venomous, but not dangerous to people,” it reads.

“Mangrove snakes are a shy, arboreal species that are active at night. There is little chance of seeing or coming in contact with this snake due to its timid, secretive nature but if you see it, please notify a staff person.”

This is not the first time the Bronx Zoo has lost a snake. A deadly Egyptian cobra escaped in 2011 — and wasn’t found for a week.