I looked at the "Alien Race" table in the DB to watch diplomacy progress with a new NPR that shot at me, and I have not noticed diplomatic relations trending back to non-war while our races are not in mutual detection range of each other.
This prevents the scenario referenced in the C# Diplomacy Changes, where a race shoots at another's survey ship, but as long as you avoid contact, relations trend back to non-war. Instead, they are remaining static according to the DB, across many construction periods.
Once I put a fleet back in mutual detection range, relations started trending back to normal. This is obviously going to be a problem if they keep trying to shoot at every ship I try to bring in diplomatic sensor range. Perhaps relations should continue trending towards baseline, regardless of whether you are in mutual detection?