I noticed a lot of different solutions and tried all of them, NONE of them worked except this one.
First I'd like to document the issue.
1. Purchased a NEW (late 2014) magic mouse, used flawlessly with an older (mid 2011) MacBook Air.
2. Purchased a NEW 2015 MacBook Pro, sold the Air and paired the existing magic mouse
3. Had the issue of the Mac randomly disconnecting the mouse once every 30 minutes - 1 hour. Very strange because the bluetooth keyboard stayed connected.
4. Definitely NOT corroded terminals in the battery compartment as the mouse was new
5. Was able to use the trackpad and reconnect the mouse without moving it, so it had NOTHING to do with loose / wiggling batteries.
6. Tried the 'piece of paper' trick and new batteries, no change in the issue.
7. Tried deleting 'Bluetooth PAN' from network settings - no change
8. Tried disconnecting Mouse and re-pairing/ restarting - no change.
ALAS the solution:
Turn off bluetooth, DELETE your bluetooth preferences file and restart.
Finder > Go > Mac HD > Library > Preferences
Find
'com.apple.Bluetooth.plist'
if you're nervous about deleting this, rename it with '.old' at the end - when asked if you're sure, say yes.
Restart your computer, OS X will rebuild a new .plist file.
Delete the old one, if you haven't done so already.
Fixed!