Welcome. That seems like a odd one. Have you tried calling a dealership yet? It may (should) know something. If not, this may help:

2015 XTS - Trunk will not unlock...

My 2015 Cadillac XTS trunk will (randomly) just not unlock..... no matter what I do....key fobs don't work, trunk button doesn't work, door button doesn't work. The only way to unlock the trunk is to crawl through the back seat and pull the safety release handle. Took it to the dealer several...

www.cadillacforums.com

 

Pervies, here is a long shot, but perhaps worth a try. You say that you crawled into the trunk and pulled the emergency release cord and the trunk still didn't open. Maybe the latch is releasing, but the lift assist cylinder is too weak to lift the trunk. Try going back in there, but this time push up on the trunk while pulling the emergency release cord and see if it opens.

 

Quick test of the latch solenoid - ensure it's even working. Press the trunk-mounted exterior switch and listen for the solenoid latch to activate. That may help give you a starting point to troubleshoot further.

CCC

 

Here's another idea. From outside the car, and with one hand on the top of the trunk lid and the other hand under the lip of the trunk lid, alternately push down and then pull up rapidly back and forth in a sort of "rattling" fashion. While doing that, have someone else try to open the trunk.

 

Another thought, valet mode:

Valet Mode

I see where it can be set, but I can't find in the manual what it does in that mode.

www.cadillacforums.com

 

Considering the previous post regarding the Valet mode, does the glove box open? Wouldn't Valet mode lock that as well?