The @Transactional annotation is the new Grails-y way of controlling transactional behavior in a service. It uses an AST transformation to essentially wrap public methods (if added to the class) in a GrailsTransactionTemplate. So you get code that looks like this:

public void reject(Invoice invoice)
  {
    Reference invoice = new Reference(invoice);
    CallSite[] arrayOfCallSite = $getCallSiteArray();
    GrailsTransactionAttribute $transactionAttribute = (GrailsTransactionAttribute)ScriptBytecodeAdapter.castToType(arrayOfCallSite[9].callConstructor(GrailsTransactionAttribute.class), GrailsTransactionAttribute.class);
    GrailsTransactionTemplate $transactionTemplate = (GrailsTransactionTemplate)ScriptBytecodeAdapter.castToType(arrayOfCallSite[10].callConstructor(GrailsTransactionTemplate.class, this.transactionManager, $transactionAttribute), GrailsTransactionTemplate.class);
    $transactionTemplate.execute(new _reject_closure5(this, invoice));
  }

The real body of the method is in the reject_closure5, which ends up calling a method named "$tt_reject(...)".

Clover doesn't realize that the code originally under the reject method has essentially been relocated to the $tt__reject method.

Given that @Transactional is a core part of Grails (starting in 2.3, I believe) and the recommended way to control transactions going forward, this is probably something that should be addressed in Clover core.