I'm afraid there's no simple way, unless you can ask the programmer. As I said, it's much more likely to be a snippet than a plugin. When you edit the snippet, you can use the browser's search function to look for the name of the placeholder.
If you're good at MySQL, you could construct a query in PhpMyAdmin to look for for the placeholder name in the 'snippet' field of the snippets and the 'plugincode' field of plugins.
Here's another way to go that I think should work. Create a new snippet called FindPlaceholder with this code:
/* Find Placeholder snippet */
$ph = $modx->getOption('placeholder', $scriptProperties);
$output = '<h3>Searching Snippets</h3>';
if (!empty($ph)) {
$snippets = $modx->getCollection('modSnippet');
foreach ($snippets as $snippet) {
$content = $snippet->get('snippet');
if (strpos($content, $ph) !== false) {
$output .= '<p>snippet ' . $snippet->get('name') . ' contains placeholder ' . $ph . '</p>';
}
}
} else {
return '<p>Error: Placeholder property is empty</p>';
}
$output = '<h3>Searching Plugins</h3>';
if (!empty($ph)) {
$plugins = $modx->getCollection('modPlugin');
foreach ($plugins as $plugin) {
$content = $plugin->get('plugincode');
if (strpos($content, $ph) !== false) {
$output .= '<p>Plugin ' . $plugin->get('name') . ' contains placeholder ' . $ph . '</p>';
}
}
}
return $output;
Then create a resource called Search with this code in the Resource Content field:
<h2>Placeholder Search</h2>
[[!FindPlaceholder? &placeholder=`placeholdername`]]
Replace 'placeholdername' with the name of an actual placeholder (just the alphanumeric part, no tags or tokens).
Save the resource, then click on "View" at the upper right to look at it in the browser. It should report any snippets or plugins holding that placeholder.
You can replace the placeholdername, "Save," and "View" to see other placeholders, but I suspect that they'll all be in the same snippet or plugin.
One catch is that if the snippet or plugin loads a class file and the placeholder is set in the class file, it may not appear in the snippet itself, but it shouldn't take long to find out if the FindPlaceholder snippet is failing. It's possible to alter the snippet to handle this, but let's hope it's not necessary.
[Update] Another thought. The placeholders look like they might be part of an eCommerce package, so if there is one, they would probably be set in one of its snippets or a class file loaded in one of them.
