I asked, because im nice like that and this was the answer I got.
it's not something that is used on a regular basis, but sometimes on high threat details. You will park (stash) a vehicle at a remote location or at a back door of a venue to be used in the event of an emergency evacuation (it can also be used to slip celebs out of a location while everyone is focused on the limo). Generally, you park it early and you lock the car and it would be the type to blend into the environment, like a mini-van or non-descript sedan. It's like a worst case scenerio plan. Hope that helps.
I know that 2 such vehicles where placed days before out side the vauxhall factory in Luton in preperation for the reaction to the closure and mass redundancies by a wiley ol' Irish Chauffeur/BG.
and as luck or threat would have it, they were needed.