I had no idea that was against workplace etiquette.
Years ago my boss had a 4:30 - 6:00 meeting every Friday to make sure no one left early. I thought it was kind of cool, made me feel valuable.
Now that I'm in his position, I don't do that but I'm always there until at least 6 on Friday and available in case anyone needs anything. Plus working later Friday means less work Sat and Sun. That being said, a lot of guys are there later than I am on Fridays.
Again, I had no idea it was normal to leave early Friday.