Prince Guillaume attended the Zermatt Summit in Switzerland. The conference was held 3-5 June. It seemed to be a humanitarian leadership conference. Archduke Rodolf was also in attendance.
I know that The Hereditary Grand Duke is often listed as simply "Prince Guillaume" in many news sources. The article does not specify if it is The Hereditary Grand Duke or his uncle. However, The Hereditary Grand Duke was in Shanghai until 3 June and his flight back to Europe was around 24 hours long (he needs a vaca, right?). Therefore, I conclude that it could was Prince Guillaume not The Hereditary Grand Duke who was present...if you were wondering.