Its simple really, let them continue until they slip up, they always do at some point and then boom, we boot them.
In my youth there were 2 former german POws that lived in my town and settled there and had families. Its eminently possible that the guy posting is a relative of someone similar, good at german, having knowledge but not specifics.
Just keep your eyes on them. What I will say is that its good to have a native german here, although Ive a rudimentary understanding of the language through 3 tours there Im not an experten.
So just monitor and report for now guys…