When closed Modest will spawn a background daemon for checking email based on your stated prefs. So you can check one moment, not do anything, and the next it will be running. You should change your preference to no automatic checking to stop it respawning in the background.
The biggest issues I've had with variability in batter life have always been traced back to the type of WiFi access point that I'm using. Those with WMM Powersaving mode work 100% better than those without. Even a small amount of surfing without WMM and the battery takes a wallop.