We try to encourage our clients to treat carbon bikes just like any other bike.
Paint chips, stones thrown up off the road and minor impacts will not cause a failure in a high quality carbon frame.
Its a lot tougher than we think. There is impact resistance built into the design.
The very nature of how the composite works tends to stop crack propagation at the microscopic level.

I think with a metal frame we assume paint chips and small dings are no problem so we tend to ignore them. With a carbon frame we are unfamiliar with the material and how it works so we tend to be hyper sensitive to any small thing.

The guy at the shop doing the tap test was on target. I have seen surveyors of high end ($1mil+) racing yachts do the same thing.
However, simply a change in sound is not a death sentence. There are other reasons the sound could change. Your frame has no history that would make me assume it is de-laminating. If it were a no name "black plastic" frame MAYBE...but the odds of a modern Trek just delaminating with no history is so remote.

Here's how I approach suspected "cracks" in frames
What is the history?
If there was a big impact that would have damaged ANY material frame....we assume some damage may have taken place.
If it's minor impact, typical stuff....and we are all standing around wondering if its an issue....its probably not an issue.
Keep riding it. Do the tap test on it from time to time to see if it changes over the next month or so to make yourself feel better. Likely it will not change.
If it changes and becomes more apparent...then explore your other options but this is a pretty rare case.

Keep in mind what happens when your bike falls over.
The pedals, wheels, seat and handlebars take the impact. Imagine even the bike falling out the back of a truck onto the road. Its going to tumble over the wheels, handlebars, saddle and pedals. All the while dissipating energy. The odds of the frame taking the first and hardest impact is remote. If it did take a direct impact here...it would kill steel, aluminum, carbon and probably titanium, so it wouldn't matter the material

The place you can hurt a carbon frame is clamping. This is where things are different than ferrous frames.
If you are clamping your frame into a workstand or old school thule rack, adjusting stems on steertubes or seatposts without a torque wrench you are asking for trouble (ie the hamfisted Trek mechanic a couple years ago)

An interesting read is Craig Calfee's white paper on Carbon. It's out of date but an informative read anyway, I wish he had time to update it. Calfee knows his stuff when it comes to CF composites. Re the Ruby...If Craig fixed it, I wouldn't worry about it failing ever.