'Key not Detected' displays on every Rainy day.

This is about my Prius 2010. On every rainy day, car panel turns on to display "Key Not Detected" automatically, that in turn drains the battery during the night. This happens when car ignition is turned off. But when I sit inside the car with key in my pocket, the message goes away and car acts normally.

Can anyone advise me what's going on, please? Why it happens only on a rainy day / night?
Thank you.

 

I have checked all the batteries, ( key fob and Car battery are good and fairly new). By further investigation I have figured out that the sensor on the door handle, once wet, short circuits to signal the door is open electronically, not physically. The panel lights turn on as a result (automatically) and look for the Key. When key is not found it start displaying ' Key not Detected' message on the dashboard. This remains ON over night (8-9 hrs) till I go to the car in the morning. By then the battery drains sufficiently and doesn't start the car.
Solution: Currently, I have solved the problem (temporarily) by fooling the system. I left the key fob inside the car so that ' Key not Detected' message does not turn ON, as the key fob is right there and locked the car manually with the key. I guess a technician will have to look into it further to see how does water get in the sensor or maybe I will find a local solution for it too, eg. putting a transparent tape on the sensor.