What usually happens is that someone reports you. If they find out you stayed for x amount of time with someone you aren't related or married to, and someone has complained about that, then it's assumed you were having illicit relations and you are deported and in the most extreme cases lashed.
And no, it doesn't just apply to Muslims.

"We submit to the majority because we have to. But we are not compelled to call our attitude of subjection a posture of respect." Ambrose Bierce