Detection with false_negative value

I’ve been querying traffic sign detections with the Mapillary API, using as a basis. I ran into a query that returned a feature that didn’t actually have object detection characteristics, and instead of having a value of traffic–sign–class (for whichever class it might belong to) the value was instead ‘false_negatives_wi1bpUvz7imEO4oXe1e2zn.’ I haven’t found any info on false_negative (or false_positive) classes, could someone please help me to understand what the detection value here means? Are there any other similar values that I might eventually encounter, such as false_positives, unknown, etc.?

Here is the json info for the feature
{‘type’: ‘Feature’, ‘properties’: {‘captured_at’: ‘2018-07-14T21:20:12.803Z’, ‘image_ca’: 290.5864562988281, ‘image_key’: ‘BBQCkXNHL4a9GP2Wx9kV0d’, ‘image_pano’: False, ‘key’: ‘x6bz00lkv8slvylx6a369g’, ‘value’: ‘false_negatives_wi1bpUvz7imEO4oXe1e2zn’}, ‘geometry’: {‘type’: ‘Point’, ‘coordinates’: [-77.48635362645298, 43.11247652981471]}}