Propane and butane are heavier than air. A leak doesn't drift away: it sinks, and inside a hull it settles in the bilge, waiting for a spark. A gas locker keeps the cylinders and regulator in a space sealed off from the boat, with a drain at the bottom that lets leaked gas run overboard.
ABYC A-1 covers LPG systems. As Soundings summarises it, the locker must be vapour tight to the hull interior, open from the top, sit above the waterline and drain overboard from its bottom to an outlet lower than the locker floor and above the waterline.
What a surveyor checks
Open the locker and look at what's in it: cylinders secured, nothing else stowed. Check the lid seals and the locker doesn't open into the boat. Find the drain at the bottom and follow it to its outlet: it must run downhill all the way, with no low spots, and come out above the waterline. Check where the hose leaves the locker is sealed, and look for a gas detector and solenoid.
In the report
"Gas locker, drain outlet in the transom below the static waterline, locker floor lower than the outlet. Leaked gas can't drain. Relocate the drain above the waterline with a continuous fall before the gas system is used." See LPG locker drain.