Flip this around.
What if your messages are landing in others’ queue, and you don’t even know it..
What if your messages are landing in others’ queue, and you don’t even know it..
You should be collecting metrics on the basics of the way your service operates and in a steady state at scale even a 1/2% drop in messages should be readily noticable and likely monitored.
It is far from certain that any application has such a "steady state", most of the ones I've worked on sure don't. There are obviously ways to analyze things and correlate enqueued and dequeues, but it is far from as simple and black and white as you suggest, especially with truly distributed systems and unknown cause of the reported behavior.
Heck, we don't even know if the messages are being "dropped" or just duplicated.