LPG Cylinders are filled to a level which will provide a remaining vapor space, adequate to facilitate the thermal expansion of the liquid when subject to a temperature increase.
In filling stations, each time a cylinder is filled, it must be have a soapy water leak check by the filling operator in accordance with the standard operating procedure for decant filling.
There is no danger with transporting small volume of LPG cylinders that are appropriately restrained and correctly filled. However, like all other fuels, LPG should be treated with care although you carry it inside a steel cylinder, you should not throw it away.
LPG (Liquified Petroleum Gas = propane and/or butane, or their mixture) has the advantage of clean burning, high heating value and the fact that it can be kept liquid (i.e. at a very low volume when compared to the volume of the gas) by holding it slightly pressurized. There are many different designs and sizes of such pressure tanks from the one way lighter to LPG tankers.