Their non-logging-based investigation got them to the point that they were aware the problem occurred when a companion fell from a great height while the player was on their ship:
> The only place in the game when a companion is present but not in the active party is when the player is on their ship
> Eventually we figured out that "undamageable" does not mean "invulnerable" -- they can't take damage from attacks but can still get hurt from other things
> One of those things: falling a great distance
> The problem with that is that there are no spots in the player's ship that are high enough to result in a lethal fall
> So now we had to figure out how companions were mysteriously ending up way above the level
> I looked into tons of theories
This does look like logging would have caught it. They knew exactly what they were looking for: companions reaching strange heights while the player is on their ship.
> The only place in the game when a companion is present but not in the active party is when the player is on their ship
> Eventually we figured out that "undamageable" does not mean "invulnerable" -- they can't take damage from attacks but can still get hurt from other things
> One of those things: falling a great distance
> The problem with that is that there are no spots in the player's ship that are high enough to result in a lethal fall
> So now we had to figure out how companions were mysteriously ending up way above the level
> I looked into tons of theories
This does look like logging would have caught it. They knew exactly what they were looking for: companions reaching strange heights while the player is on their ship.