Things like this make the best shop geeks go a little nuts...
Make sure that things stay surgically clean as you go through this...
It becomes a process of elimination when you encounter issues like this and chain ring bolts are often the cuplrit... if they have been greased and torqued to spec they should be okay.
If the crank isn't worn and has also been torqued to spec we can eliminate this as well... for now. The debate on whether or not to grease spindles is an epic one and would say that if you are using a torque wrench it does not present the problems it can if you are applying Sasquatch strength and go a little too far.
I'd be looking at the UN54 and bb as a possible source of the creak as even when you can't feel any play in a cartridge bb, that does not mean that something can't be wrong with this and one also needs to check it's fit on the bike.
I'd start by removing it, make sure the threads in this and the bottom bracket shell are squeaky clean, grease the threads and reinstall the bottom bracket. In a lot of cases the bb shell could benefit from having the threads chased with a tap. I do this as part of any bb overhaul as quite often the threads could be a lot cleaner than they are and not every shop preps a frame properly.
If this does not address the issue I would replace the cartridge with a fresh one and see what happens.