Sensor keeps beeping

It sounds like one of the sensors mounted in your bumper is bad or you have something obstructing one of them, mud, snow, etc.

unplug them one at a time until the noise stops, that will be the defective one

What do you mean "parking sensor"?
Do you mean the parking brake is on, or is the seat belt not being used?
You should be able to tell where the sound is coming from.

The easiest possibility is that the proximity sensor thinks something is constantly very close to your bumper. Assuming you don't have any weird stuff hanging off your car that could confuse it, the sensor is likely damaged.

I am not sure what noise you have. it sounds like its either a reverse alarm which sounds when you are backing up. that's what it is supposed to do. If it is an alarm... it may be telling you that you have a reverse light out.

clean off the sensors on the bumper

Sensor has failed/Malfunctioned or needs to be reset take to auto electrician for further diagnosis. I hope this helps.

because the prius is such a nightmare to work on,i would suggest you try to live with the noise or park it where you dont have to reverse.you cannot just bypass the buzzer.......

Try to clean rear parking sensors. Check also electric cables and connections, clean if necessary. Commons faults are sensors out of order. If one or more sensors faults you hear only a continous constant noise when you put in "R" gear. You need a VAS, VAG-COM, Bosch KTS, ... diagnosis scantool. The factory parking system will be scaned from errors with this instrument, and you will find exactly which sensors are out of order (for example: rear left center sensor, rear right side sensor break off, short to ground or short to plus).

Try to clean rear parking sensors. Check also electric cables and connections, clean if necessary. Commons faults are sensors out of order. If one or more sensors faults you hear only a continous constant noise when you put in "R" gear. You need a VAS, VAG-COM, Bosch KTS, ... diagnosis scantool. The factory parking system will be scaned from errors with this instrument, and you will find exactly which sensors are out of order (for example: rear left center sensor, rear right side sensor break off, short to ground or short to plus).

I find the sensors only activate when you are very close to an object behind you. At about 5 inches they beep constantly. Further then five inches they beep slower and slower until they stop at about 8 inches from something behind you. If you are very close to an object lest then 2 inches they will beep constantly. Also when you start your car and put it in reverse and you hear a beep the reverse sensors are not working. The best way to test them is to turn the key to the start position but do not start it. place the lever in R get out or have someone walk in back of the car, you should hear the sensor go off. Also, there might be a situation when one or two are not working. Follow the procedure above for test but have a stethoscope mechanic type, which most auto parts stores carry. and listen to each one individually. If one is not working it will have to be replaced. The control module for this system is in the trunk next to the spare tire. Some of these cars had a leaking trunk problem in 02 if the module is wet, get a new one. If it hasn't failed it will when they get wet. Finally the sensors should be heard inside and outside the car.