Can someone tell me what this is?

Question

It's on a 2015 Kia Optima. Hanging down right behind the front bumper. It had a metal bracket that looks broken.

Answer 1

That will be the ambient temperature sensor. It is used by the air conditioning system.

KIA’s description from the weblink -

Kia Optima: Air Conditioning System / Ambient Temperature Sensor

The ambient temperature sensor is located at the front of the condenser and detects ambient air temperature. It is a negative type thermistor; resistance will increase with lower temperature, and decrease with higher temperature.

The sensor output will be used for discharge temperature control, temperature regulation door control, blower motor level control, mix mode control and in-car humidity control.

If the ambient temperature is below 2.0°C (35.6°F), the A/C compressor will stop.