Rainbows, If you have a Facebook account, search for the local interest and local neighbourhood watch groups.
they are quite good at spotting and reporting on vehicles behaving suspiciously.
From what I saw around North Sheffield before I moved away, I'd be surprised if it showed up locally in one of the scrapyards, though it's obviously worth a look.
I regularly saw transit pickups around our way collecting "scrap" but found it hard to believe that the piles of new kids bikes, good condition garden furniture, and fine cast iron ornaments, could all have been voluntarily "donated". I'm even aware of central heating radiators, both brand new, and even still with the paint drying, have been liberated.
Personally I've lost various large lumps of metalwork, that wasn't visible from the road or even a cursory glance around the back.
Also a full car engine, 4 motorbikes over the years, and someone tried nicking a Landrover gearbox whilst I was there fitting it, similarly the bonet, whilst I was working on the engine on a different day.
I have it on good authority that some of this was collected by a particular extended family, who were known to be rather violent. A little later, this moved on to a different community "litter collection group", also very threatening, perhaps moreso. Be very careful if you have to challenge them.
It's hard to mooch around scrapyards these days, but it's good that, officially at least, they're not allowed to purchase for cash.
I honestly believe that leaving real scrap out on the pavement only encourages these lowlifes to proliferate.