"Is it a game that somebody is playing but we just don't know how to play it?"

It’s the case of the empty sauce bottles.

Dozens of A.1. steak sauce containers have mysteriously been popping up inside Ohio’s Avon Lake Public Library, leaving the staff very puzzled. “At first, when you find one you think, oh, what’s in here—was somebody sneaking booze in the library?” Jill Ralston, the library’s public relations and marketing coordinator, tells PEOPLE.

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Since a a security guard spotted the first bottle tucked in a pile of newspapers on Jan. 11, a total of 30 bottles have been found. They are stripped of their labels but, with a little investigating, the staff was able to identify the familiar sauce bottles. “We measured it and we looked online and we compared and determined it’s an A.1. bottle,” says Ralston. “They’re dishwasher cleaned out but you can still catch a hint of A.1. in them.”

The bottles were found in the adult fiction or non-fiction sections—never in the children’s book sections—every few days and the prankster has an M.O. “For the most part, they’re hidden behind the books laying down on their side at the end of the row with the cap showing,” she says.

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While the bizarre mystery, first reported by The Chronicle Telegram, sounds straight out of a John Green novel (Margo, is that you?!), Ralston says a teen culprit can be ruled out. “We do think they’re being left during the first half of the day because we seem to find them after 3 o’clock. So we don’t think it’s a student. That much we kind of surmised,” she says. Local police have even gotten involved to no avail. “Nobody knows anything. We’re stumped.”

“We’ve tried to think of different things,” Ralston continues. “Is it a game that somebody is playing but we just don’t know how to play it?”