Luke said that other people tried to test Jesus (ἕτεροι δὲ πειράζοντες), by demanding or seeking from him (ἐζήτουν παρ’ αὐτοῦ) a sign from heaven (σημεῖον ἐξ οὐρανοῦ). There were other instances about people looking for signs from heaven, but not within this context. In Mark, chapter 8:11, the Pharisees asked Jesus to show them a sign from heaven, since they wanted to test Jesus. Heavenly signs had been common among the prophets to prove their authenticity. In Matthew, chapter 16:1, both the Pharisees and the Sadducees came to Jesus. They wanted to test Jesus also. They also asked him to show them a heavenly validation of his work. Later in Luke, chapter 11:29, there was also talk about the sign of Jonah. Thus, there was a continual attempt to test Jesus, by asking him to give some heavenly signs. Do you try to test Jesus in your life?
While both Matthew, chapter 12:39, and Luke, chapter 11:29-30, perhaps using a Q source, said that Jesus would only give them the sign of Jonah, Mark said here that Jesus was not going to give them any sign at all. He said that Jesus sighed deeply in his spirit (καὶ ἀναστενάξας τῷ πνεύματι αὐτοῦ). He asked them why was this generation seeking a sign (λέγει Τί ἡ γενεὰ αὕτη ζητεῖ σημεῖον)? With a rare solemn proclamation (ἀμὴν λέγω ὑμῖν) in Mark, Jesus told them point blank that no sign would be given to this generation in this convoluted phrase (εἰ δοθήσεται τῇ γενεᾷ ταύτῃ σημεῖον). Sometimes miracles were considered heavenly signs, but Mark continued to call miracles works of power and not signs, as other gospel writers sometimes referred to them.