We use this to mean 'I'm surprised to see you here.' and/or 'I didn't expect to see you here.' The expression uses the older meaning of the verb 'fancy', which is to 'imagine'. If you say 'Well, fancy that!' it means 'What a surprise. It would never have imagined such a thing'. By the way, we often use 'Fancy seeing you here' in a sarcastic fashion, if two people meet in a place where they have often bumped into each other before - suggesting that it isn't a surprise at all.

Thank you to all of you.

It has an quaint ring to it that's untranslatable. I would leave the English untranslated but provide an explanation in the language you're translating to along the lines that Su.Ki mentioned.

None of the above. I cant imagine you are here

"I didn't expect to see you here " is the closest meaning