If you enter your MP number into a *A itinerary that includes UA flights, the UA flights show up on UA.com when you log in.
(I know this because I used my MP number for a Expedia-sold BMI-stock AC-codeshare UA/AC-metal itinerary and it showed up on UA.com with no other action on my part.)
So, if someone messed up and put in your MP number rather than theirs, you would see their reservation including any associated passenger details.