According to the search results, Nespresso offers various gifts and promotions to its customers, but there is no clear information about a specific "Nespresso mystery gift." However, there is a Reddit post from 2022 that mentions an offer that includes either a blanket made of recycled coffee ground or a ceramic mug as a free gift with purchase. Additionally, Nespresso offers a welcome gift for new customers, which includes a $10 off coffee code that can be redeemed at checkout or in-store. Nespresso also offers an exclusive gift on the anniversary of becoming a Nespresso & You Ambassador, which is €10 if you have an Original machine, or €15 if you have a Vertuo machine or both. On their Instagram page, Nespresso showcases their products and promotions, but there is no mention of a "mystery gift". Therefore, it is unclear what the "Nespresso mystery gift" refers to, and there is no definitive answer available in the search results.