Can I record audio in my kitchen when I leave, if I suspect my infant's safety is at risk?

Question Details: My now ex-fiancé and I live in the same residence with our 7 month old. We met in recovery meetings. I am 4 years and 4 months sober, alcohol only. She used IV street drugs, cocaine and heroin. I have suspected her relapse for months now. I saw her leave our residence without our son recently and neighbors also tell me that people come here every time I leave. When I asked her about it, she denied both. Is it unlawful by federal or state law to hide a smart phone in the kitchen of my residence to record audio when I leave home as I fear my son's safety is at risk? The kitchen is the entrance we use most.

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