I'm not underestimating it so much as disregarding it completely. A nobleman stands out, simply put, from the rank and file of his hirelings. It's like comparing the destruction of a main battle tank to the death of an infantryman. And as each knight or lord brings an entourage of scribes, lesser nobles, advisers and yes-men with them, there are plenty of eyes that are only watching what happens to their patron, and could easily report what happened to them to the army at large. You do get a message when someone is captured, or wounded, or killed in the battle; this request is more about doing something with that rather than introducing new ideas.
You don't get to pick through the gear of the fallen but you do get to collect your wounded. A wounded nobleman would be easy to spot, just look for the sun shining off his platemail. As for assuming that any missing noblemen were captured by the enemy, typically the enemy would send word of any captures, whether to illicit a ransom or simply to flaunt it. A nobleman is not just another soldier, they're an invaluable instrument of the realm. They are part of an intricate distribution of power that trickles down from on high and are not to be disregarded as the average footman may be.
If you think you can reliably keep track of a single melee combatant from a distance, no matter HOW different their armour or equipment may be, then yes you are disregarding the realities of melee combat. A standard falling is an event that has a reasonable chance of being spotted from a distance though.
