Starship is going to be fueled with methane/oxygen cryogenic liquids. Prior to launch vehicle will spend some time fully loaded on the launchpad. Although methane is lighter than air at normal temperature, at - 161.5°C (boiling point of liquid methane) it's 30% denser than air. It has to be heated additional 30°C to become lighter again, which not going to happen fast. Therefore it's possible to buildup around the Starship in significant amount even if refrigeration is considered, sealing is never perfect and leakage can happen. Liquid methane expands 590 times. Considering it's lower and upper explosion limits with air are 5.3-15.5%, ignition of the rocket can easily trigger a detonation. It may seam minuscule, but one cubic meter of gaseous air/methane (90/10) mixture is equivalent in explosion power to 190g TNT. How SpaceX plans to deal with this kind of explosion hazard around Starship?
Boil-off of methane is piped to a Flare Stack and burned off so it won't be vented unburned into the atmosphere. You can see a good picture of the flarestack on the right in the image of Starhopper fueling below:
(Image credit: NasaSpaceflight.com (bocachicagal)