Admittedly I was only thinking of like safes and lockboxes, stuff you could probably open without the key anyway given a bit of time

I doubt there's any forgotten houses or cars in my family's junk collection in any case and I have no use for old bike locks and padlocks, they'd binned anyway whether I found a matching key or not

If you have a decent photo of the side of the key, you can make a pretty good guess at the bitting code, which is essentially the instructions for making a copy. Here's an example of the process: https://www.youtube.com/watch?v=JoRyv4ANhM4