I’ve done this very thing using a microwave radar sensor instead. Positioned it behind the glass and used the standard PIR module which works without any additional configuration (just need to tell it the correct GPIO pin as standard). It was a RCWL-0516 sensor and there’s loads on ebay.
MagicMirror² v2.20.0 is available! For more information about this release, check out this topic.
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RE: Camera / PIR behind mirror glass