i done the same thing with a classic rangieI bought a cylinder with some lpg in (it was full!).
Didnt find out until i i started to take the valve out.
I sealed it back up (eight allen bolts in a circle, so just tightened them back up).
I then took it onto the wasteland at the back of my house, about 800yds from the nearest anything, and unsealed it.
Wandered off and went back an hour later.
Remember LPG is heavier than air so will tend to sink and could accumulate down any nearby drains.