I sent a feedback with this exact issue. I know I have had "shady" guests that have purposely booked for one rather than the actual number of people staying. I have also had guests that have booked for one but when I send them a quick message to change the reservation they gladly accept and send an apology that they meant to book for more.
In one of the "shady" instances the only way I knew that there were exra people before they left is because I live next door to my rental and heard a dog barking just outside (whereas normally the dogs that bark in the neighborhood are across the street or farther away).
I looked outside to see if maybe there was a stray dog and saw my renters. My rentER I should say, since they booked for one. They had a dog, 2 adults, and a child. Instead of going out and confronting them I sent them a kindly message via Airbnb asking them to confirm the amount of people staying etc. Keeping the conversation away from being accusatory allowed them to confirm in an Airbnb message that they had the extra guests. I asked if the child was going to be staying in the room with them and their reply was "she wanted to sleep on the top bunk in the guest room but I wouldn't let her" so I assumed they would only be occupying the one room.
This wasn't the case. The little girl apparently felt like messing up not only the bottom bunk in the room I didn't think was going to need to be cleaned later, but also the top bunk. I pay my cleaning service extra for each bed (hence why I charge more for extra guests) so I sent them a request for money. They ignored it for more than a week and I had to get Airbnb involved.
Luckily I had the messages there for Airbnb to see, so they quickly got me the extra money from the guests, but it shouldn't have had to get to that point. Granted, these guests would probably still have done the same thing, even with software enhancements, but it would have been harder.