The Quran

فَلَمّا قَضَينا عَلَيهِ المَوتَ ما دَلَّهُم عَلىٰ مَوتِهِ إِلّا دابَّةُ الأَرضِ تَأكُلُ مِنسَأَتَهُ ۖ فَلَمّا خَرَّ تَبَيَّنَتِ الجِنُّ أَن لَو كانوا يَعلَمونَ الغَيبَ ما لَبِثوا فِي العَذابِ المُهينِ

Muhammad Asad

Yet [even Solomon had to die; but] when We decreed that he should die, nothing showed them that he was dead except an earthworm that gnawed away his staff.¹ And when he fell to the ground, those invisible beings [subservient to him] saw clearly that, had they but understood the reality which was beyond the reach of their perception,² they would not have continued [to toil] in the shameful suffering [of servitude].³

This is yet another of the many Solomonic legends which had become an inalienable part of ancient Arabian tradition, and which the Qur'an uses as a vehicle for the allegorical illustration of some of its teachings. According to the legend alluded to above, Solomon died on his throne, leaning forward on his staff, and for a length of time nobody became aware of his death: with the result that the jinn, who had been constrained to work for him, went on labouring at the heavy tasks assigned to them. Gradually, however, a termite ate away Solomon's staff, and his body, deprived of support, fell to the ground. This story - only hinted at in its outline - is apparently used here as an allegory of the insignificance and inherent brittleness of human life and of the perishable nature and emptiness of all worldly might and glory.
Al-ghayb, "that which is beyond the reach of [a created being's] perception", either in an absolute or - as in this instance - in a relative, temporary sense.
I.e., because they would have known that Solomon's sway over them had ended. In the elliptic manner so characteristic of the Qur'an, stress is laid here, firstly, on the limited nature of all empirical knowledge, including the result of deductions and inferences based on no more than observable or calculable phenomena, and, secondly, on the impossibility to determine correctly, on the basis of such limited fragments of knowledge alone, what course of action would be right in a given situation. Although the story as such relates to "invisible beings", its moral lesson (which may be summed up in the statement that empirical knowledge cannot provide any ethical guideline unless it is accompanied, and completed, by divine guidance) is obviously addressed to human beings as well.