The NSA ANT catalog of spy devices (leaked by Snowden) also includes cheap analog retro-reflectors: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;NSA_ANT_catalog" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;NSA_ANT_catalog</a>