Locus app disappearing quietly on Samsung phones
After moving from a Note 2 with CyanogenMod to a Note 4 with original Samsung ROM, I noticed that Locus sometimes would simply die quietly throughout the day. Obviously, track recording would stop and I would lose data until I looked at my phone again and noticed that the Locus app was gone. Not even the "run Locus as service" option in Locus fixed this.
I believe the reason for this is Samsungs hyper active and completely brain dead memory management, that goes around and kills apps at will. You can read excellent details on this here:
It also explains how to disable this major Samsung nonsense, but only when you're rooted.
If you're not rooted, I think you're out of luck. I have no idea if there's anything Locus can do to prevent from being killed. It will obviously depend on how many other apps you use in parallel, what things happen on your phone, and probably on the moon phase too. If you've never experienced any Locus disappearances on newer Samsung phones with original ROMs, consider yourself lucky. But if you've had the same problem, this might explain the reason. My troubles stopped when I disabled the samsung memory management as explained in the above link.