Whether there was a company policy, ethics, or not, it would seem prudent to not consume alcohol at lunch. It's probably not a big deal for an office worker other than if you made some decision or design that failed, if someone knew you had been drinking at all at lunch, that could call your judgement into question whether you were only one drink or 10 drinks. IMO it's a perception issue more than anything.
I drove school bus all the way through college, granted that is a different situation (operating a bus with kids, etc.) than a desk job, but I never drank inside of 8 hours prior to driving. Sure, it would have been out of my system, there are no vehicle laws or company policies against drinking the night before. But had I been in a crash or had a mechanical issue I missed on a pretrip inspection, if somebody had seen me drinking that would have been a judgement impairment question then and me saying "I only had one" vs. the eye witness saying "Well, I only saw him drinking, but don't know how much" No proof either way unless you can produce your bar tab and the tabs of your party.
Maybe thats over cautious, but nobody can question you then. A "No, I was not drinking at noon" is a lot easier to deal with than your word vs. another witness, coworker, or client.
