20% overnight battery drain of iPhone 4S

I have got my new iPhone 4S for about two weeks. I noticed several times that the battery went down about 20% overnight (around 8 hours) while it was in the airplane mode.

This happened either when the battery was fully charged or when it was partially full. This also happened even when I made sure that the only app that was running in the background was an alarm app while the phone was in the airplane mode.

Why did the phone use so much battery while it was in the airplane mode with only an alarm app running in the background?

Thanks in advance for answers!

I sent the phone to an Apple Authorized Service Provider. They kept it for observation. I may get it back within 48 hours.

The Apple Authorized Service Provider called and told me that they could not find any problems with the hardware.

They said that they put my iPhone 4S into the airplane mode for more than 2 hours and observed 5-10% drain of the battery, which is consistent with my finding.

They first tried to persuade me to take the phone as it is. After I insisted for an answer for too much drain of the battery, they offered me to replace the battery. Since the phone was precisely on the 14th day of purchase when I brought it to the service provider, I insisted on an exchange. They then said that they had to send the phone back to the factory. When I agreed to sending the phone back to the factory, they backed off and said that they would observe the phone in the airplane mode for 24 hours.

I will call the apple online support when I get access to the serial number of my phone.

I will keep updating this thread till the resolution of the problem. I may start a poll for the battery drain of an iPhone 4S in the airplane mode.