Steam Summer Adventure - Guest Pass ?

Hi,

When I crafted a Summer Adventure Badge today I got a Steam Popup which came up basically saying I received some kinda Guest Pass for a game. There's nothing telling me what game it's for and when I clicked on the "Manage Guest Passes" it brought me to my gift inventory but there's nothing new there and no guest pass to be found.

Anyone else got something like that and what are guest passes anyway ?