It means the ticket was not full used and has "expired" -- as you might expect.

I don't really see how this will help. It is highly likely your BP was not scanned or recorded on the flight and there is nothing you are going to find in the ticket details that will explain that.

You need to convince an MPSC agent that was an error and you indeed did take the flight. It is not unknown for this to happen and all agents should be familiar with the possibility of BP scanning problems. Have in your hip pocket some proof of travel (credit card receipt for rental card?, restaurant?, lodging?, taxi?, ... and offer it to the agent, the agent likely will not actually request it.