What is the difference between ,,They are somewhere here.,, and ,,They are sure to be somewhere here.,, I do not fully understand, why somebody use this ,,sure to be,, and how it change the meaning. And of course in which case I should use the second sentence instead of the first one?

See a translation

1st sentence: I think you mean "they are here somewhere" you know they are there, maybe because you left them there. 2nd: You are very confident they are there because that's where they are supposed to be, maybe you didn't leave them there yourself but you can't think of where else they could be so they must be there. There is a slight worry/urgency/doubt in the 2nd one. maybe. The difference is almost non existent, there might not even be a difference. I would just use the first one and don't worry about it "sure to be" is just filler.