Leo may have once smoked (his nephew gave him an ashtray he made at camp and although a young nephew might have given a non-smoker an ashtray, it indicates a possibility that he once smoked [#3]. Also, he smoked when he relapsed into drinking before the last debate of the first Bartlet campaign for President [#309]. And, as Nikolas Cassadinne reminded us, Leo smoked in "Angel Maintenance" when the President's life was in danger on Air Force One.
Most people's office doors are usually kept open including the Oval Office. Almost half the time, though, Leo's three doors are closed, which is more often than the Oval Office door.
"I can't remember the last time I got home before midnight." [#13] C.J. but everyone else demonstrates the same thing.
"Nothing like surrounding yourself with the best and the brightest" --- the President, sarcastically, when he's told the staff didn't know it was raining [#19].