I take the picture, slide the lock, and then put the phone back in my pocket. I do not exit the camera app or close the lens cover.
When the camera app goes into standby I believe it continues to look for a gps lock to tag the photo. However, after the gps location has been acquired and the photo has been tagged the camera app will try to hold the gps lock thus draining battery. I came to the conclusion that the gps lock still runs when the camera is in standy because when I take the camera out of standby to take another picture the gps position is immediately known (pushpin icon not crossed out).
I used this system at Legoland at the weekend. 58 of the 60 photos that I took were tagged. The battery died (after 10.5 hours use) as I took my final 'goobye Legoland' photo...perfect timing