In the ancient time (maybe Roman era) when a war finishes, when an army killed (yield) another army, generally speaking what do they do with the corpses? My question is specified to a war in a ground or forest where a castle or kingdom isn't near; do they have the time to burn them all? Is it safe after an exhausting battle to burn them or maybe to bury them all together? (The winning army especially needs to manage its dead soldiers.) If they just left the bodies wouldn't that cause diseases? Perhaps a soldier of the defeated army is slightly injured and pretends he's dead, and when he feel safe he can leave and tell what happens.
The short answer is: loot them.
Once the bodies had been looted, they would be left to rot. There is a famous passage in Herodotus concerning the Battle of Pelusium which had occurred about 75 years previously:
On the field where this battle was fought I saw a very wonderful thing which the natives pointed out to me. The bones of the slain lie scattered upon the field in two lots, those of the Persians in one place by themselves, as the bodies lay at the first- those of the Egyptians in another place apart from them.
Many similar accounts of old battlefields could be listed.
As far as "escapees", that did (and does happen). In some cases the winning army will have soldiers make sure all of the enemy are dead and this can be combined with looting activities.