A couple things to get out of the way first:
Who are the "their" to whom Jesus refers?
Luke 11:
As I understand you, your question above concerns "their" in Luke 11:17. I'm going to put some notes in parenthesis. If I add a word, I'll italicize and underline it. If I disagree with the way a word is translated and think it meaningful to whatever degree, I'll strike it. If I bracket some words, it's because they are added by the translator. I typically use the NKJ because I'm used to it. I'll get into the Greek if I think it's necessary. If you want something better defined, let me know.
NKJ Luke 11:14-17 And He (Jesus) was casting out a demon, and it was mute. So it was, when the demon had gone out, that the mute spoke; and
the multitudes (crowds) marveled. 15 But
some of them (some from the crowds) said, "He (Jesus) casts out demons by Beelzebub, the ruler of the demons." 16
and others (from the crowds), testing Him (Jesus),
sought they (others from the crowds)
were seeking from Him a sign from heaven. 17 But He (Jesus), knowing
their (could be the others from the crowd who were testing Jesus - could also include those from the crowds alleging Jesus was casting out demons by Beelzebub) thoughts, said to
them (from the crowd): "Every kingdom divided against itself is brought to desolation, and a house [divided against] a house falls (literally "a house upon a house falls" - a house falls upon a house - kind of like a house of cards).
"Their" is those from the crowd. Since it says Jesus "knowing their thoughts" I might go with "their" being the ones who were testing Him. IOW, Jesus knew what they were doing in testing Him by seeking a sign from Him. "Their" could also be referring all of them, His accusers, and the ones seeking the sign, and even most to all of the crowds. Jesus is speaking among a crowd of people and in effect, He's speaking to all of them, and the issue was, who is Jesus?
The reason I struck "sought" and changed it to "they were seeking" is because the original wording is painting a picture that this is a very active situation and they were continually seeking this from Jesus. It's not a picture of a few who sought, but more of a dynamic crowd situation with some chaos typical in crowds. The dynamic is some wondering if He is Messiah (seeking a sign) and some accusing Him of being demonic, a false Messiah, a false prophet, etc...
If you search through the chapters leading up to this, you'll see that up through Luke 9:20 Jesus is not letting people spread that He is Messiah/Christ. All this chaos in part is because His notoriety had been spreading (Luke 9:11) and people were wondering and likely and lively debating who He was.