I called Apple support. After ruling out wrong usage (“Did you kill all applications, you know the iPhone4 can do multitasking ?” and “Have you switched off WiFi and location services ?”) I was advised to backup the phone, reset it completely and see what happens.
Well, guess what. I backupped it, reset it, charged it completely and after 3 hours standy I was still with the 100 %. Sooooo…. seems to be a software issue. Only problem is: I did not change any software on the iPhone when the problem occured.
At the moment I am setting it up again, installing only the software I use (you might know those App-corpses: Installed out of curiosity, used only one ?).
Problem is – what about the data, all my tasks in OmniFocus for example ? They are in the backup which contains the “faulty” settings which cause my battery to go dead within 10 hours of standby. You know, I tried that stunt once before calling Apple. Backup, reset, check…. seems to be ok…. restore from backup. Problem again present.
Mails, pictures, music, podcasts – everything is on the Mac and can be restored. But all the information from my Apps…. my jogging history in 321run… in the “faulty” backup.
Ok, at first I am glad that a solution could be found. Now I have to see how the iPhone behaves tomorrow and then I can worry about getting my data back.