I would try analog Mux at the input configured as DPDT (double pole double throw), connected to ADC_SAR differential input. Input 0 is connected to sensor high output, and input 2 is connected to sensor low output. Input 1 is connected to the ground and input 3 to sensor low. The ADC is always in differential mode.
For signal measurement inputs 0 and 2 provide differential path to ADC input. For sensor presence testing, the Mux is switched to inputs 1 and 3. ADC will return ''negative" voltage, but that is correctable in software.