With proceedings getting underway at the FIA's headquarters in Paris on Thursday morning, Horner joined a host of representatives from Mercedes and Pirelli.
It is not clear at the moment whether Horner is at the hearing simply as an observer or will be called upon to provide evidence.
Horner has been among the most outspoken against Mercedes' actions in testing a 2013 car, with his outfit having lodged the original protest against Mercedes in Monaco.
In Canada a fortnight ago, Horner made it clear that his unhappiness was directed solely at his on-track rival, and not towards Formula 1 tyre supplier Pirelli.
"I think the issue is nothing to do with Pirelli," he explained. "I can understand fully why Pirelli would want to test.
"I think they wanted to understand the product for the following year.
"The issue was they asked for a car to be used, and it is the entrants' responsibility to ensure that that car complies with the rules.
"Our issue is not to do with Pirelli; it is to do with the current entrant. They [Mercedes] broke the rules with the current car."