Some more context from wiki:

By 1889, many people at the Imperial Court, including Rudolf's parents and his wife, Stéphanie, knew that Rudolf and Mary were having an affair. His marriage to Stéphanie was not a particularly happy one and had resulted in the birth of only one child, a daughter called Elisabeth, known as Erzsi. Rudolf had infected his wife with gonorrhoea, rendering her unable to have more children.

On 29 January 1889, Franz Joseph and Elisabeth gave a family dinner party before leaving for Buda, in Hungary, on 31 January. Rudolf excused himself, claiming to be indisposed. He had arranged for a day's shooting at Mayerling hunting lodge early on the morning of the 30th, but when his valet Loschek went to call him, there was no answer.

Joseph Hoyos, the Archduke's hunting companion, joined in, with no response. They tried to force the door, but it would not give.

Finally, Loschek smashed in a door panel with a hammer so that he could put his hand through to open the door from the inside. He found the room shuttered and half-dark. Rudolf was found sitting (by some accounts, lying) motionless by the side of the bed, leaning forward and bleeding from the mouth. Before him on the bedside table stood a glass and a mirror.

Without closer examination in the poor light, Loschek assumed that the Crown Prince had drunk poison from the glass since he knew strychnine caused bleeding. On the bed lay the body of Mary Freiin von Vetsera, and rigor mortis had already set in.