It could also be triggered by specific locations (leaving town, just before getting major boss monsters, the first time you enter certain locations, etc).
If saving in Riftrunner takes more than a couple seconds (which is likely), I would rather have a configurable reminder to pop up either once or periodically if the game has not been saved in a certain time frame. This could be similar to the broken equipment symbol which tells you something needs to be repaired... maybe a scroll and/or feather pen symbol.
