The original owner probably sold it on, de registered and the new owner didn’t register. I’ve done that beforeI've run some of my frame numbers through the Bike Register database out of curiosity, and never had a hit - even on a bike that had a prominent warning sticker on the top tube saying it was registered! That I find very odd, unless the frame number wasn't recorded correctly - in which case even if the bike was stolen there would be no way of knowing or proving it one way or the other. That bike was £12 so I won't worry unduly about it. If I was buying bikes in three or four figures I would take a less casual approach, but I generally don't pay much over £20 for a used bike, so it doesn't represent much criminal profit potential.
My point was the btwins aren’t 25 years old. Hence my saying it seemed cheap for a good condition modern bike