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Just enabling them, not necessarily using them, do they eat up battery life?

Find my iPhone does use battery on standby as it requires push notifications from iCloud. It's the reason why when you use FMI it can initiate the locate command to your iPhone's software.

Siri, I guess not so much unless you initiate it.
 
Just enabling them, not necessarily using them, do they eat up battery life?

find my iPhone imo is only useful if you have a passcode and location services on. On an iPad you might as well just leave it off, since if you lose it you'll probably never get it back. If you could disable hardware buttons on an iPhone it would be even more useful so if it's lost nobody can do anything with it
 
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