04-11-2016 23:12
04-11-2016 23:12
04-12-2016 00:43
04-12-2016 00:43
You're right in that the Surge should monitor you quite happily without your phone around and only needs it to sync.
I can only guess that it was unable to detect your heart rate for some of the time. This can be down to several things as explained in this article:
https://help.fitbit.com/articles/en_US/Help_article/Heart-rate-FAQs/
Basically, if you can see the heart rate on your Surge then it should be there on your dashboard after you sync. It's worth keeping an eye on it to see if you can spot when it's not picking up the heart rate.