Hi
Thanks for posting and sorry to have taken a while to respond.
In the absence of anyone 'on the ground' I will (likely) make direct contact to satisfy my curiosity 
The person who contacted me via my website contact form explained that they had seen my pictures on the wall at these premises though when I asked could not recall just what they had seen, even though all of the pictures sold are displayed on the website. They explained that one of the contacts as the premises had given them a business card with website details on it.
As in my OP, I have made not direct sales to the USA (Australia yes but that was the only international sales) and certainly no business cards........but every picture was labelled with a "title" and my contact details.
Subject to some sort of confirmation that I have been 'advised' correctly I can only surmise that one of the people in the UK I sold multiple prints to have sent them a USA friend or associate. Alternatively, they have been screen grabbed (though they have a light watermarking) and printed with "their" concession as to the source being a card they can give to interested parties/persons.
Having someone 'eyeballing' them could have given me a little insight/feedback before reaching out to the business owner contact I was made aware of.
So, I just need to send a polite enquiring email to see if there is any feedback or answers forthcoming Just need to think on what to say?
