$.post returns incorrect empty string

You'll receive [object XMLDocument] in your jQuery $.post's data ...

var $xhr2 = $.post( 
 options.init_src,
 {
  'lookupId': theValue,
  'field': options.field,
  'primary_key': options.primary_key,
  'db_table': options.db_table
 },
 function (data) {
  $input.val(data); // textbox receives [object XMLDocument]  
 }
);

..., when you return empty string from your controller:

public string Caption(string lookupId)
{
 return "";
}

In order to prevent that problem, just add "text" parameter to your $.post method return type

var $xhr2 = $.post( 
 options.init_src,
 {
  'lookupId': theValue,
  'field': options.field,
  'primary_key': options.primary_key,
  'db_table': options.db_table
 },
 function (data) {
  $input.val(data); // receives "" string now.  
 },
 "text"
);