In Customer Trips, a user can only see those that belong to their MKT area.
I've been through the documentation and it says: "To include interactions in your customer journeys that don't have a marketing area assigned, make sure your user role is also assigned to a blank marketing area. A blank marketing area is represented by a space enclosed in single quotes ('').
For more information, see Contacts and Interactions "
How can I assign a blank MKT area to a user's role?