Well, sometimes you might be experiencing some odd unexplained drain event, but not near a PC (or at least, not near one that you can install ADB on). I've had that happen sometimes myself. Probably in this case there is no right answer once you're talking about user interfaces.
The time gets accounted against SOMETHING, but I'm not sure what and haven't figured out yet. Since I have an SGH-I777 and not a GT-I9100, I haven't had a chance to poke at the ICS leaks for the I9100 to see how they behave with some of the classic situations of high Android OS usage on those devices. (boo for the legal corner cases of leaked firmware...)
As an example, if you happen to know someone with an SGH-I777 or GT-I9100, try:
ping -i 5 <WLAN IP address of the device>
Watch Android OS usage skyrocket, and battery level start to drop rapidly (faster than holding a permanent wakelock)
Then install a custom kernel that includes the above patch, and do the same thing
Battery level drops rapidly, but no good way for a device user to figure out why.
Real-world non-synthetic non-device-bug examples of situations that might cause the above wakeup patterns/drain: Skype, Windows Client Backup broadcast traffic, Dropbox Lan Sync probes every 30 seconds, misbehaving UPnP AV (aka DLNA) systems with lots of SSDP broadcast traffic. Not sure what a good way to help the user diagnose such issues is - on Gingerbread the only way is root + tcpdump + send results to someone who knows how to read a pcap. Or turn off wifi - but since the BCM4330's standby drain is so low, a user SHOULD be able to leave it on all the time and get better battery life than routing background data over the cell radio.
Real-world bug example: The UCKK6 build for the SGH-I777 has a bug where Broadcom's BT-AMP function intermittently goes insane, causing the BCM4330 to wake up the CPU as often as once per second. As far as the user is concerned, "bad OTA, makes battery drain on wifi". No way to diagnose/confirm without tcpdump, no way to use tcpdump without root, no way to get root without technically voiding warranty. http://forum.xda-developers.com/showthread.php?t=1409513&page=4
includes a paste of some data from a tcpdump cap. (Apologies for the profanity - but it's just another thing that reinforced my hatred of Broadcom.)