Can Angels appear in human form to help out in a situation and then disappear again when you are in great need of help?
Check out Judges 6 from verse 10 onwards, an angel appears to Gideon who calls the angel “sir” – very polite boy!
Although the angel looks like an ordinary person, Gideon wonders if it is really God who is talking to him and telling him to go and clobber the Midianites. And, being Gideon, he decides it’s best to ask for a sign. So he goes and prepares an offering, after getting the “angel” to promise to stay.
When he gets back, the “angel” whacks the meat of the offering with his stick and it burns to a frazzle in seconds. The angel then disappears.
Now you should notice that it is only at this point that Gideon finally cottons on that it WAS the LORD who was appearing to him.
He does the traditional “Oh no! I’ve seen God face to face! I’m going to die!” routine, then he doesn’t die after all, presumably because it’s not very easy to follow God’s instructions and clobber Midianites if God has just killed you. And God knows this, you see.
Similar stories occur in other passages where angels appear without people realising, so check out Judges 13, the story of Manoah and his wife, (where you can enjoy the “Oh no! We’re going to die!” routine again). Also see the story of Lot, Gen 19.
So, though for most accounts of angels, it is fairly obvious what is happening, (like it was for the shepherds when Jesus was born) it’s not always so obvious. The writer to the Hebrews tells us to be hospitable to strangers, “for by so doing some people have entertained angels without knowing it”. Hebrews 13:2