If the player is offline for more then 5 minutes. They will be teleported to the start of the instance.
If the player and everyone else in the instance is offline for more then 30 (or 60?) minutes the instance will soft reset. Check if this is a new instance or merely if all trash have respawned.

Currently we assume that being teleported to the start of the instance is due to party leader resetting while offline. Which should still be default?

If the party leader does have the addon. We can correctly determine if the instance has been reset while inactive.