Almost certainly they knew from USAF records the individual aircraft numbers and fairly accurate position of the P38s near Kulusuk on the East coast of Greenland. With most of the aircraft on the ice cap (and there are a heck of a lot of them), it was a case of being caught out by the weather or fuel - usually both, cos there wasn't (still isn't) a reasonable diversion available from Sondrestrom on the "spruce route" delivery trail to the UK in the war.
Sondy was known as Bluey West 8 during the war, with the more famous Bluey West 1 at Narsassuak down on the southern tip of Greenland. Most of the aircraft that didn't make it were ditched on the ice, and the crews rescued. Its pretty flat once you get away from the coastal areas and pressure ridges.
I recall when searching in 65 for a missing Mooney that had attempted to cross the cap, one of the aircraft "found" a B17 100 miles SE of Sondrestrom just sitting on the edge of the ice, resplendent in its US Army camouflage with guns in the turrets completely whole, like the B29 Ki- Bird. It happened to coincide with Arctic Aviation Week being run by Life magazine and Life managed to dig out the crew from all over the States and flew them back up there for a nostalgic look-see.