Yes I have witnessed one in Vietnam back in the summer of 2012. My father's sister (my aunt) and her Caucasion American husband were staying at his house in Saigon (Ben Thanh district). A police officer stopped by the house stating that someone reported foreigners were staying there. My dad, my uncle, and myself were the only ones home at the time. My aunt and her husband were out shopping. My dad said he was friends with the head of police station and will have a meeting with him to discuss it. I did not attend the meeting, but was told afterwards everything was fine. That was the only police visit I witnessed even though I have visited Vietnam many times and lived there for several months. If I had not personally witness it, I could argue that such things don't happen because you don't usually hear about it. But I go back to my statement that just because one hasn't experience something, it doesn't mean it doesn't happen.