is it feasible that the crew of the 767 didn't notice it?
Yes, feasible depending on the severity of impact. I remember an incident in MAN about 7 years ago with a similar mismatch in size, the crew of the larger aircraft (the moving one in this case) felt an shudder but it was not immediately obvious that there had been a collision. The stationary aircraft in that case were immediately aware of the impact.
Link for
AAIB Report.
sr