Continuum is storing scm passwords in the database in plain text.
If the 'use cached credentials' checkbox is checked, it should use the provided userid and password for the initial pom retrieval, and then discard them.
(Continuum has the ability to use svn credentials that have been pre-cached on the build server, but when you add a project the first request for the pom is not a svn checkout, it's just an http/https GET.)
Workaround: periodically remove the credentials from the database:
update PROJECT set SCM_PASSWORD = "";
update PROJECT set SCM_USERNAME = "";