@Yiwei3 Unfortunately, there isn't a designated dock for reporting discrepancies like this. If you're personally handling the check-ins, you can request that the guest show ID or otherwise match their profile credentials in order to get access to the property. But if you're using a keycode/self-check-in, this can easily get a lot more complicated. It doesn't appear that Airbnb has a ready solution for the occasion when a guest who's entered your property via keycode doesn't seem to be a match for the profile in the booking.
Since Airbnb is not actively providing screening tools, it's crucial that you have a system in place to keep control of your own property and know who's coming into it Whether that means doing in-person check-ins, having a co-host monitor the space on your behalf, or a more high-tech solution like external surveillance connected to your smartlocks, you should set up whatever security protocol best serves your needs and disclose it fully in the listing. If someone who you didn't authorize to access your property is able to get in, there's not much Airbnb can do for you - you've gotta have a system in place to make sure that doesn't happen.
But if you are handling check-ins directly and the account holder does not arrive, and you don't feel comfortable accommodating the person there in their place, you can ask Airbnb to nullify the booking without penalty. Generally this means you will lose the entire payout for the booking, but sometimes it's just more important to be cautious about who gets the keys to your home.