Thanks for the comments
@Sandman - that particular PWD is Only used on B4X. We have a policy on PWD creation so every site's log in credentials are unique in that respect. And the USR is also unique to B4X come to think of it...... All USR are just email addresses as per the site request - but all PWDs are unique. (It's not a very long list, but it's a lost none the less.)
For completeness, they don't say that the combination of username and password has been breached. They only mention your password. So even though you might have
MyVery1337Password as your password for this forum only, it's entirely possible some other individual have that exact same password - which has appeared in a breach. Which might trigger that alert.
All in all, I wouldn't worry too much. But I would also change my password, just in case.