How can I get zero values to differentiate no detections versus a non-functional sensor?

Add "filter.value.gte = 0" to get zero values.

Using the Reporting endpoint, you can get zero values from when no people are detected under the sensor by adding filter.value.gte = 0.

All sensors send us data even including zeroes, whereas a non-functional sensor will not send data at all.

{
    "group_by": {
        "order": ["space_id"],
        "raw": true
    },
    "window": {
        "every": "5m",
        "function": "max"
    },
    "filter": {
        "start": "-1d",
        "value": {
            "gte": 0
        }
    }
}