Attempted connections to local device through pub IP without port forwarding

Having attempted logins on 22 on a non port forwarded box from a variety of remote IPs.

Mainly wondering as to how they can find the ipv4 address of box X through wan and the public IP and attempt to login (without traffic from 22 being forwarded to box X, by the router)

This may be something obvious in regards to networking as a whole but im just looking to understand and am eager to hear your thoughts

Thanks