Trigger enemy is treated as (None)!
1. Try this out in the last build [679]
Set two rooms with Dungeon Boss enemy flag. (These rooms on a same level number in a DMap) One room with the Trigger enemy and the other with nothing or any enemy that you can kill off.
One room with Trigger will have a screen flag, Enemies->Secret.
Enter the other room and kill off (if there are no enemies,...) then go to the room with the Trigger. Trigger is unavailable because all the enemies in the other room with the Dungeon Boss enemy flag had been killed or they aren't any enemies, rendering all rooms with Dungeon Boss enemy flag not to have enemies in the same DMap level. Thus, you'll hear the Secret SFX.
2. Now try it in the current build [692]. Enter the room with the Trigger and, you will hear the Secret SFX. The Trigger is buggy in this build.