The burger was carried into space last month aboard a helium balloon and was missing from its container when the balloon fell back to earth and landed in a tall tree in Massachusetts.
Renzo Lucioni told The Boston Globe he and his colleagues had to hire a tree trimmer to go up the 100-foot tree and retrieve the deflated balloon, but the sandwich was gone. Lucioni told the newspaper a lucky squirrel might have dragged it off.
A camera aboard the balloon had gone dead before the burger disappeared so it offered no help in cracking the mystery. The camera, however, did catch great images of the hamburger, donated by the burger chain b.good, soaring into the clouds and on to the upper reaches of the atmosphere.
The YouTube video went viral with more than 150,000 views in less than a week, the Globe said.
Lucioni and his team want to follow up their landmark achievement with another balloon mission. The current goal is the first burrito in space, but nothing official has been nailed down.