Solution
Go to:
[1. input/Edit] - [1.1. Sows] OR [1.2. Boars] - [1.1.1. Arrivals/removal]
Click on 'Departed animals' (pig icon with an 'X').
Locate the departed animal by clicking on 'Search' (pig icon with a magnifying glass).
Click on 'Edit'.
Delete the date in the 'Departure date' field.
The animal is now back in your records.