I’ve deleted a few posts from this topic, which were derailing this conversation into off-topic territory, and kept getting repeatedly flagged. Let’s focus on sharing our experiences with missing stamps, if you have any.
As for me, it’s happened maybe once or twice — both in postcards I sent, and postcards I received. It’s relatively rare though, and not something you should be alarmed about, @amagata. 
Not here. Most people are not interested in collecting stamps.
I just sent a Bessie Coleman postcard with a Bessie Coleman stamp. The receiver contacted me to let me know the Bessie stamp was missing. I use a lot of vintage stamps with the water-activated glue. I use a paintbrush to moisten the gum and diligently ensure it sticks over the entire surface of the stamp. There was another stamp on the postcard that was applied the same way, so it was obvious that the Bessie stamp was deliberately removed. How can we stop this?