The bike commuter I knew who did this was a contract programmer. His wife and kids lived in Houston and he had a cheap apartment 300 miles away in Fort Worth where he worked. He drove home each weekend. His 25 miles r/t bike commuting was just how he filled his few hours of free time each workday in a cheap healthful pursuit. Many of the contractors on that project actually lived quite frugally so some were living in RVs, sharing apartments, no cable/internet.

So he wasn't leaving the car around the office just to run errands - but it did end up that that car was available to do so if he chose.