Hidden tags are robust and efficient security systems for protecting assets but their identification requires nondestructive techniques. Here, a contactless, efficient, terahertz (THz) investigation protocol based on the electromagnetic interaction between THz radiation and materials with subwavelength features is presented. On top of an opaque, lossy dielectric substrate a metal “open tag” is patterned as reference. This contains features that are visible to the naked eye, but subwavelength in the THz range. An identical dielectric substrate is then used to cover the metallic pattern and fabricate a “hidden tag.” Both tags are then scanned using a THz time‐domain spectroscope (TDS). The collected spectra are used to evaluate different metrics, which provide valuable pixel‐specific information for sample imaging. While the most common imaging techniques are demonstrated to be unsuitable for decoding hidden tags, a …