Two devices fitted with GPS trackers were dropped from a helicopter off the south coast of Iceland a year ago.
Both floated west, passing the coast of Greenland towards Canada before drifting back east across the Atlantic.
One was found on Tiree this month. The other has floated north in the direction of the Faroe Isles.
The devices' journeys could be tracked on a website set up by an Icelandic TV science programme which was available to the public.
The experiment was designed to highlight how rubbish dropped in the sea does not disappear but becomes a problem for people living on coastlines in other parts of the world.
Icelandic scientist Ævar Þór Benediktsson said both devices were released from the helicopter at the same time.
He said winds were a big influence, adding that it remained unclear where the second "message" would end up.
Islander Rhoda Meek recovered the device that washed up on Tiree. She carefully packaged it up and posted it back to Iceland.
A "message in a bottle" involved in a scientific experiment drifted off in a different direction from one that was washed up on a Scottish island.