I took my car to the dealer with this problem... At first, they couldn't replicate the problem and asked me to pick the car up. Right before I picked the car up they washed it for me and the beeping started. They think that they found the problem:
"We found the problem with the car. There was a water leak at the base of the windshield, that would allow water to get to a module. We have been able to seal the area with out any problems. However we are going to have to replace the module. We have special ordered the module and asked to get it overnight if possible. We will know in the morning. I know this could hamper your plans. We are trying to make your schedule work to get your vehicle back home."
The first time it happened to me it wasn't raining (and I don't think that it had been raining the previous 24 hours), so I'm not 100% convinced that it is fixed (I should be getting it back tomorrow).
This glitch is too prevalent and too sporatic to be a function of a water leak. I don't think that's going to solve the problem. It's most likely a software issue having to do with communication across the car's CAN bus (discussed at length in several threads) and my guess is that regardless of what the dealer replaces you'll still see the issue until the SW is updated.
Having said that, keep us posted and let us know if it does the trick!