Both me and my gf have had this. Both have HTC one on the 3 network as well.
It's only a guess but it could either be when u have no signal and for some reason it's roaming to another network so it can make emergency calls... Pointing to a software issue.
The other thing I have noticed that may have something to do with it is it only happens to me when I am in power saving mode with cpu saving ticked. As soon as I untick power saving it switches back to normal.