Everyone has been there. A pat to your your pocket or a look in your purse. That sinking feeling hits. The phone is absent. Oh crap. Where is it? Damn it, damn it, damn it. My preciousssss!
Sure, the phone's 'magic find me beacon' is probably active and that's great, but it likely won't help much if there's no nearby computer and a friends' phone is all that is available. Signing into accounts on someone else's phone sucks. It takes forever and if two factor authentication is turned on, well...welp.
Note: Using google's android Device Manager app from someone else's phone, you have a fairly easy 'guest login' option, which is great, unless you have two factor authentication turned on (which you should).
As long as it's not a big deal for a few people being able to know where you are all the time (spies/ninjas, you should probably stop reading now), using Google+'s location sharing feature, one can selectively share their phone's exact location with specific people or circles.
The setting is available via Google+ > Settings > (your account) > Location Sharing:
When your friend hits the location link on your Google+ profile page, they'll be brought to a map of where the phone is and when the location info was last refreshed. Google+ is smart enough to ping the phone when the request is made to get a refreshed location.
Here's how it looks to your friend when shared:
This recently saved my bacon as I was on a ski retreat with my company. I lost my phone on one of the runs down the hill. Zipper didn't fully close. After my "Oh S#!t" moment on the next lift up and thanks to having my exact location shared with one of my close friends with whom I was skiing, we were able to retrace our last run, and watch the blips from his phone and my phone get closer together on the map. We found the phone quite quickly sitting gently in the snow on the top of a mogul. We were obviously lucky that we had signal on the mountain and I had shared my location with a few of my closest friends.
Without this setting, I would have had to
A. gone down the mountain, logged into Android Device Manager to figure out where it was and went back out. It would probably have been picked up by someone else who may or may not have turned it into lost and found,
B. scoured the hill slowly retracing my steps over and over like a dope until I either give up and hope someone returned it to lost and found, or
C. spent a while trying to login to Android Device Manager on my friend's phone, which sucks. Logging into your personal account on someone else's phone is basically my least favorite thing ever (and would have been fruitless due to 2 factor authentication).