Better to use bagof/3 or setof/3. As Lee Naish and Richard O’Keefe have pointed out, findall is unsound if the Template doesn’t capture all the free variables in the Goal.
bagof and setof allow you to mark variables as existentially qualified (although you might want to define an auxiliary predicate for clarity), and also allow backtracking over the other free variables. The downside of using bagof and setof is that if you want “no solutions” to result in [], you need to wrap it: (bagof(X, pred(X), Results) -> true ; Results=[]).