That PBY, Strawberry 9, was at the end of his 700-mile leg when he sent that message main body. It could be Nagumo's carriers are following the large ships he reported.
11 seconds sound clip from the Midway (1976) movie soundboard.
You can hear this line at 01:00:48.862 in the Blu-ray version of the movie.
Quote context
[...]
- I wanted to tell you myself, Joe. You were right, it's Midway.
- Hot damn! You know what I mean, Admiral.
- Thank God Fletcher joined Spruance in time.
- Another one from Strawberry 9, sir.
- Well, they may be just beyond his horizon, Admiral.
- If they are, Fletcher's in the wrong place. He's too far north.
- He'd have to move right now at flank speed to get within striking distance before sunset.
- But if Nagumo's carriers are where we figured coming in behind this storm front...
- Fletcher'd be caught between two forces.
- Yes, if we let him head for that main body sighting.
- Via northwest, as previously anticipated. Signed, Nimitz.
[...]