Deliberate eavesdropping is certainly very difficult; however, a few anecdotes coming!!
In my parish, the end of the bench people use for the confessional is about 4 feet from the confessional door - which until recently was poorly-fitting and not completely sound-proof. Anybody sitting right at the end of the bench (which people waiting for confession tended to do) could certainly hear voices coming from the confessional, although they would probably have had to strain a bit to hear what was being said. And a couple of years ago I had the experience of actually being in confession, hearing voices outside, and the door opening behind me - and the person coming in actually stood there with the door open, turned to their friend and said “There’s already somebody in!” before closing the door. Fortunately I’d completed my confession but it could easily have been otherwise. The worst thing from my point of view is that many years ago I used to be an altar boy, the sacristy was next to the confessional and the priest would hear confessions before the Saturday night Mass - and sometimes forgot to close the door on his side of the confessional, and also the sacristy door would be open!
Sometimes I think I’m one of those people things “just happen” to, something will happen and when I tell people they just say “Well that’s never happened to me!”