Do not forget to entertain strangers, for by so doing some people have entertained angels without knowing it. (Hebrews 13:2)
If every stranger you met were a potential angel, how would you treat strangers? Pretty well, I would guess. We typically treat people we know with a level of respect and honor. What this suggests is that we treat those we don’t know with the respect and honor you would give to an angel.This idea of entertaining angels harkens back to the Old Testament when Abraham entertained two strangers who turned out to be the angels who informed him that he and his wife, Sarah, would have their long-awaited son even in their old age. It has been said that Abraham kept the flaps up on all four sides of his tent so that he could spot travelers in all directions and offer them hospitality. Part of this was the custom, but the custom was driven in part by the anticipation of angels. Abraham probably entertained numerous strangers and travelers before two of them actually turned out to be angels.
I’m okay with angels. Believing in angels doesn’t make you weird. It doesn’t mean you are seeing things either. What if, in fact, we saw angels every day and didn’t know it? Angels in the Bible sometimes appear as bright, shining beings that strike fear into a person because of their extra-terrestrial nature, but sometimes they appear as normal human beings. They would be strangers because they would have taken on human form for a particular task—usually to deliver an important message or perhaps protect someone from danger. It’s kind of cool to think that you might pass a few angels today; and if you are friendly to strangers, you might even entertain one
If we were anticipating angels more, we would be more alert to those around us at all times. Might not be a bad way to live. Keep the flaps up on our tent, so the speak. Even if we never did know, we would live better and be better people.
John Fischer The Catch 
The movie City of Angels explores this idea a bit (prior to the love story) and I have always found it intriguing.
This is wonderful! My husband and I had an encounter with an angel and our lives have never been the same. I wrote a book of southern short stories and that event is in the book. We had car trouble. Out of the blue a red truck pulled up beside us. The man got out, didn’t say much and presented the part we needed. He worked under the hood quietly. My husband asked if he could pay him something and he replied, “Just help someone in a little red truck sometime.” He got in his truck, pulled out and disappeared into thin air. We both saw it, so we knew we weren’t seeing things. About a year later, Eddie saw a red truck on the side of the road. He stopped and helped the man who began a conversation about the Lord.