A nobleman biting it would be a major event on the battlefield as their unit scatters and the enemy overruns their position in the line. Unless they're a Hero, they typically don't lead from the front. And noblesse oblige requires that one treat a noble opponent's corpse with respect according their station, though depending on your realm, mileage there may vary. Either way, the conquering army rarely sets up camp right on the field they took since the ground has been churned up by thousands of boots and hooves, instead withdrawing back to their camps to allow both sides to collect their casualties. This is why you have wounded men after a battle, who may be healed and return to battle, instead of the losing army simply subtracting all casualties.
A knight wears distinctive armor, usually carries a personal standard of some sort and wields the finest combat gear available (Reflected in BM in that there is no equipment rating for nobility, and thus no cap to their Swordsmanship skill beyond the skill itself). Even in a heap of other bodies he would stand out.
You completely underestimate the chaos of melee fights. After the focus of the battle has passed sure you could find a wounded noble, assuming that your side held the field and scored the victory. The wounded soldiers could just as easily be walking wounded as seriously wounded requiring aid to leave the field.
Think of it this way, you can scavenge the battlefield if you lose, why do you expect you could wonder around it looking for nobles whom mostly likely would be captured by the enemy that control the field anyway.
