Yeah. See... uhhh...
When I was an undergrad at uni, there were a lot of people who I knew by face but I didn't really know well enough to have a conversation. To be polite, people would always nod hello as they passed each other in the corridor. I walked in to the public toilet, saw a familiar face and instinctively nodded. He gave me a wierd look and left and it was only later that I remembered who he was.