In the after-credits scene of Infinity War, Hill tells Fury that there was an energy signature from Wakanda just before the snap. The problem is that if, as she says, they were monitoring all satellites, it should have been picked up at minimum twenty minutes earlier (around the amount of screen time the battle gets). So why didn't they find out earlier?
Although the proximity of the statements makes it easy to get confused, Hill's team were only watching satellites in the hope of getting a message from Tony Stark, assuming this transcript is correct:
FURY: Still no word from Stark?
HILL: No, not yet. We're watching every satellite in both hemispheres, there's still nothing. [receives three beeps from her device]
FURY: What is it?
HILL: Multiple bogeys over Wakanda.
FURY: Same energy signatures as New York?
HILL: Ten times bigger.
There's no actual indication that the information about the “multiple bogeys over Wakanda” came from the same satellite monitoring they were doing in hope of finding Stark. There's also nothing that indicates Hill is getting this update immediately after the attack on Wakanda began.
Given how huge they are, it makes sense that their energy signatures are ten times the size of that put out by Ebony Maw's ship. Its brief appearance over New York, concurrent with Tony's disappearance, is probably the only event in the movie that Fury and Hill were aware of until now.
Hill is simply getting this information around the same time as Thanos snaps his fingers, rather than immediately. She's an important woman. She's not spending her time checking raw satellite data personally, and you don't text her constant updates the second you learn anything new. You look into it, find out what you can, and give her a succinct report. That takes a little time!
The "signature" is likely that of the Infinity Stones being used. "In New York" is a reference to the events of "The Avengers", and the signature level referenced there is that of the Tesseract (space stone) being active. The "ten times bigger" is a reference to the cumulative event of Thanos arriving, laying down a smackdown, and then the snap itself where all 6 stones are used.
There could be any number of reasons why the ships weren't detected until that moment... they didn't mention when Proxima Midnight's ship was hunting Vision in Scotland.