The SearchQueryTool is an open source tool which helps to execute Search queries for SharePoint. This article explains how to find the special property InformationProtectionLabelId.
The All Properties of item window of Search query tool (which normally shows all available properties of an object) is missing InformationProtectionLabelId:
This is because this property is not part of the normal SharePoint Search Schema and there is no good way to get the Search Schema in general. 😦
As a workaround you can directly specify InformationProtectionLabelId as managed property:
TantumPoint 