A Metal Detectorist in the U.K. Has Discovered Two Ancient Roman Cavalry Swords Once Used for Fighting on Horseback

I think what you're asking about is called Archeological context.

The basic gist is that archeologists will be able to date not just the find itself, but the soil it's in and other potential finds within the same layer of soil/around the site.

So you're right, if you just found two Roman swords on a bench in the Cotswolds, you probably couldn't infer from that that the Roman military had definitely been present. But if you find them in a specific layer of soil along with other Roman artifacts, that starts to build up context around the find. The more evidence you collect, the better able you are to put a date on when the swords arrived at your site.

We know the Roman’s had presence in England all the way to hadrians wall and a little beyond, the wall is just where they could solidify a border. Your example is certainly possible but like you said, the SS were never in Kansas while the Roman legions were in England so the comparison doesn’t really hold

Detectorists! The thing they use are the detectors. God such a brilliant bit of TV.