The internet is abuzz with talk of ladybugs.

On Tuesday, the San Diego office of the National Weather Service tweeted that it had picked up an odd radar echo that evening.

It looked like a bit of rain, the meteorologist Alex Tardy, who works at the San Diego office, told the Guardian. “But then we realized – it’s not raining,” Tardy said. The team spoke with volunteer weather spotters in the area, and one of them noticed “a lot of ladybugs around”.

Based on that information, the office sent out a memorable tweet.

The post was retweeted more than a thousand times, and both local and national news outlets caught on.

And since then, Tardy says, he and other meteorologists in the office have been working to confirm the theory – with little luck.

“It’s definitely not birds, and it’s not bats. But we’re still not sure if it’s ladybugs,” Tardy said. The radar was picking up on “lots of tiny things, about the size of large raindrops”, he noted. Those could have been bugs, or possibly chaff – bits of aluminum or other materials that the military disperses in the air in order to confuse and overwhelm aircraft-detecting radars.

The Guardian has contacted the nearby Edwards air force base and Twentynine Palms Marine Corps air ground combat center, but neither immediately responded to requests for comment.

“Because this was at night, the visibility would have been low,” Tardy noted. “And whatever we caught on the radar was pretty high up – more than 5,000ft in the air,” impossible to capture with cameras or cellphones from the ground.

“It’s too bad there wasn’t anyone in a private plane up in the air at that time,” said Lynn Kimsey, an entomologist at the University of California, Davis. “We could’ve figured it out based on which dead insects were splatted across the wings,” added Kimsey, who has worked with Nasa and Boeing in the past to do just that.

“It wouldn’t surprise me that it was ladybugs, although it seems a bit late in the season,” she said. The bright red insects are known to hibernate in clusters along California’s foothills and migrate west in the springtime.

Mobilized by warm weather, a huge group of bugs might have taken flight together or gotten swept up with the wind, she said.

But other experts noted that it was highly unusual for thousands or millions of ladybugs in California to migrate all at once. “They tend to move gradually rather than in huge clusters,” said Steve Heydon, a curator of the Bohart Museum of Entomology in Davis. “It’s not like they’re caribou or something.”

Moreover, a cool evening doesn’t present the optimal conditions for ladybugs to take flight, he noted. Ladybugs tend to be active at temperatures of about 60F (15.5C) or higher.

Weather balloons that night estimated temperatures lower than that, according to Tardy.

It’s “not impossible that a swarm could be caught in a wind stream”, said the Cornell University entomologist John Losey in an email. “A swarm this large has never been observed on radar before as far as I am aware,” though huge swarms have washed up along coasts in Egypt, the UK and the United States, he said.

Generally, swarms in the air are hard to see with the naked eye and have rarely been observed. And so, Losey said, “we may never know exactly what caused these radar patterns”.