Possible safety issue M62 j22 - 23 eastbound

Not sure if this is something to be worried about but in a field very close to the above stretch of motorway are a few HGV trailers with advertising posters on the side. I noticed when I drove past a few weeks ago that a couple of them were on a slope, pointing down towards the motorway. If the brakes or whatever was keeping the trailers in place failed, then they would be heading off down the hill towards the motorway. It could of course be that the wheels had been removed and I hadn't noticed this or they are secured in some way in which case there isn't a problem but I thought it was worth a mention. Please note the location pin I put on the map is a bit of a guess but they were definitely that side of the reservoir