Official rule text
17.2 A captured non-fighter ship or mech is returned under the following circumstances: a If the player who captured the unit agrees to return it as part of a transaction. b If an ability instructs the capturing player to return the unit as part of an ability’s cost. c If the player whose unit was captured blockades a space dock of the player who captured the unit.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 17.2