Mine didn't have a battery either but there was a slot in the case where I would have expected it to be - no worries though.
I've found that I have to give it a slight tap to "motion sensor" it on. Walking around doesn't do it which is OK with me. I was a bit concerned with the use of a PCB flap for the battery positive, but if I'm only opening this up every 6 years it should be Ok.