Many thanks to rebel_mae for sharing a photo on webstagram! Her boss’ uncle worked with Keanu Reeves and took a photo with him for her. It also looks like he got an autograph for her as well. If you look closely, you can see the outdoors area from the tent. The black tent and the vegetation looks similar to that in other photos thqt were taken on the Knock,Knock set.
Update: rebel_mae has answered an inquiry about the date of the photo. She can’t remember exactly but seems to think it was taken in 2010 or 2011. Generation Um filmed in 2010 and his hair is too long for it to have been that film so I am inclined to think it was taken on one of the 47 Ronin sets – perhaps in June or July at Shepperton.