I was wondering, besides sluggish trigger return, would a worn out or damaged TRS show any other symptoms? I.E. trigger stacking, weird inconsistent pulls, etc?
 

No, you would not likely notice anything. In the past, Beretta recommended changing it every 5k trigger pulls (not necessarily every 5k rounds - in case you do a lot of dry fire practice).

Now, comments in past years claim that the spring is made of thicker wire than itw as in the 80s. So, it will likely last longer. But, I usually just replace mine when I change my recoil spring - just so it's a non issue.

It just keeps on working until it suddenly breaks.

I think it's a reversible spring, but if you want a new one Brownells is their supplier.

Wilson Combat has chrome silicon trigger return springs and they seem to be more robust than OE springs. I change mine annually and have had zero breakages since doing so. They really are not that difficult to change.

You pull the trigger and the trigger stays back.

At least, that was my experience.

Yep most common failure I have seen with military M9s.

My 92FS was older and had the steel trigger.  I installed the Wolff unbreakable trigger return spring and haven't thought about it since.  The Wolf unit is a good design and drops right in no problem.  Instead of the OEM spring, it uses a coil spring set-up to push on the back of the trigger itself.  That's why the older all steel trigger is required.  They also have different weight springs.  I installed the standard weight spring, and it didn't feel any different at all.  

The OEM spring seems to work fine until it just snaps.  You can flip it around and use it backwards (designed to do this) until you get a new one in.  Obviously, you have to take the gun all apart to do this.  If you're getting a sluggish trigger return, it's likely some other reason than the return spring going bad.  Probably just dirty, or some old lube has turned to gunk, or brass particles in there or something simple like that.