Leaks don't just spontaneously appear. You must've moved some brushwork around and created a hole that way.
Unless you mean you're getting FindPortalSide errors, which can happen even without a leak (though it's rare), and is usually caused by excessively complicated world brushes giving vbsp a hard time. Fly through your map and convert any particularly complex geometry into func_detail, then re-compile.
Anyway, it'd be super duper helpful if you could paste the entire compile log here (not just the error in question).