An electronic device may include wireless circuitry with a transmit antenna that transmits signals and a receive antenna that receives reflected signals. The wireless circuitry may detect a range between the device and an external object based on the transmitted signals and the reflected signals. When the range exceeds a first threshold, the wireless circuitry may use the transmitted signals and received signals to record background noise. When the range is less than a second threshold value, the wireless circuitry may detect the range based on the reflected signals and the recorded background noise. This may allow the range to be accurately measured within an ultra-short range domain even when the device is placed in different device cases, placed on different surfaces, etc.