I used to farm on 1000 acres of moorland, 450 Swaledale ewe, 80 Mule ewes and all their lambs and retained gimmers (approx 100 per annum.)
They ranged extensively over large moorland enclosures, but we saw them every day (which is a legal requirement here anyway), and did expect to count them, as far as practically possible, several times a week. But yes of course, especially with the sort of terrain where sheep can hide, it wasn't always possible to get a perfect count, or to know whether, if you'd come up short, it was a miscount, or a ewe or ewes had escaped the holding or were stuck - or dead - somewhere on the plot.
So then the faithful, brilliant and indefatigable collie dogs would help, as would being able to read the landscape. The dogs would scour the ground widely and either chase errant batches back to one of the main groups, or let you know they'd a sheep wouldn't budge.
Crow activity concentrated on a far flung bit of ground would be worth investigating. As you drove around the wilder parts on the quad bike, you - or the dogs - might pick up a scent of maggots, or see or hear a concentration of flies.
In winter we'd be feeding most of the ewes, so it was easier to count heads every day as they munched the nuts we put out in a long line for them. After lambing, for a few weeks the twin- and triplet-bearing ewes would be in-bye in fields, each holding up to 50 ewes, and would be getting cake daily, so again we could count both munching ewes and playing lambs. (The number with singles would be less than 1/5 of the total, so easy enough to count them where they were out on the moorlands.)
And then every gather - crutching, sorting for tups, dosing, shearing, weaning, etc - that's when you'd do a full and accurate count, and go back out with quad and dogs to find any stragglers / escapees.
So yes, sometimes a sheep would have died in the reshes, and never been seen or missed when she could've been saved. But mostly, except very occasionally in prolonged blizzard or deep deep snow, we'd find 'em while we could still help. Or their fresh corpse if it was a sudden death.