Since I noticed strange behaviour of the battery level indicator, I have done some measurements. Here are my findings.
After charging the watch for one hour after it indicated 100%, I recorded the remaining battery charge displayed by the watch over time. During this time I did not start any activity on the watch, but the MoveIQ feature was on most of the time. Also the heart rate monitoring was on and messages from my phone are enabled. Here is a diagram of the charge level over time:
Note that the charge level drops very fast during the first few hours after being charged for 100%. Between about 83% and 57% the charge level drops at a relative constant rate of about 0.5 % per hour. After reaching about 47% it drops at a rate of approx 5% per hour, so 10 times as fast as before. The last 5% it suddenly drops slower until the watch suddenly turns off.
Note that the range between 83% and 57% accounts for 2.5 days of the 3.5 days of charge. When the watch said it had 50% charge left, it actually lasted only about 11 hours.
The curve above looks very much like the curve of the voltage of a lithium ion battery as the battery discharges. To really predict the charge level of the battery, the typical discharge curve of the battery should be taken into account. I think they call this Kalman filters.