Here's my short thought- there are so many layers to our world, and we see so few of them. For example, until van Leeuwenhoek invented the microscope, we had no ability to see germs, even though they were always there.
Similarly, in a scene in this chapter and a scene with the Prophet Bilam, something is there all along, but our protagonists do not see it until God literally opens up their eyes.
Here's the scene with Hagar in Genesis 21:19-
|יט וַיִּפְקַח אֱלֹהִים אֶת-עֵינֶיהָ, וַתֵּרֶא בְּאֵר מָיִם; וַתֵּלֶךְ וַתְּמַלֵּא אֶת-הַחֵמֶת, מַיִם, וַתַּשְׁקְ, אֶת-הַנָּעַר.||19 And God opened her eyes, and she saw a well of water; and she went, and filled the bottle with water, and gave the lad drink.|
Similarly with Bilam (whose donkey is capable of seeing the angel even though Bilam himself is incapable of it)- in Numbers 22:31-
|לא וַיְגַל יְהוָה, אֶת-עֵינֵי בִלְעָם, וַיַּרְא אֶת-מַלְאַךְ יְהוָה נִצָּב בַּדֶּרֶךְ, וְחַרְבּוֹ שְׁלֻפָה בְּיָדוֹ; וַיִּקֹּד וַיִּשְׁתַּחוּ, לְאַפָּיו.||31 Then the LORD opened the eyes of Balaam, and he saw the angel of the LORD standing in the way, with his sword drawn in his hand; and he bowed his head, and fell on his face.|
The Hebrew words are different, and it would be worthwhile to look at a concordance to figure out when the word פקח is typically used as opposed to גלה. Opening is likely different from revealing. But in both scenes, it seems clear that something was there all along (a well, an angel) and it was simply a matter of revealing what was already there to our befuddled protagonists.
This reminds all of us that "there are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."