Red Bull says a test on a 'rolling road' facility would not be illegal.
Earlier, reports suggested Red Bull had tested secretly over six days between the final Bahrain test and the season opening Australian grand prix.
The test reportedly took place at the Austrian automotive company AVL's 'rolling road' facility in Graz, involving Red Bull's troubled engine supplier Renault.
But late on Thursday, the reigning world champion team denied conducting the test.
Red Bull, however, pointed out: "The test mentioned in the article would be permitted by the regulations".
Germany's Auto Bild appears to have got to the bottom of the story.
"Auto Bild Motorsport knows that a Toro Rosso chassis without front and rear wings ran on the 'rolling road' without a flow of (wind tunnel) air.
"Therefore, the test was bench testing and completely legal. The identity of the anonymous letter writer remains," the report added.
Toro Rosso and Renault did not comment.