I don't know where you are, but if your local PD has a slow line then you might try calling that and describe it to them. I say to use the slow line because it could be nothing and it's probably not an emergency as long as no one's fucking with it; but if there's a possibility that it might be explosive then someone should check it out.

I can't really see it being anything other than a casing of some sort. Considering the shape, size and materials it would have to be something pressurised, maybe a gas. This is not typically what nitrous tanks look like (the ones people are doing in traffic for a laugh oh my god what has happened to this world). I've seen some applications of drugs but this does not look like any of them. So as far as I can tell it's either going to be something to do with construction or it's something very specific to either dogs or aviation.

Edit: considering the car park, could it be like a shell one would put on the longer end of a spark plug? They usually come in boxes without cases or shells but maybe a very particular one has this as a protective cover and someone has been fiddling with their vehicle there?