This is one of the areas where Sun Tzu’s advice in Art of War really comes into play. Know youself and you will win 50% of your battles. Know your enemy and you will win 50% of your battles. Know yourself and your enemy and you will win 100% of the time.
In todays modern age of technology it makes it very hard to remain unseen on the battlefield for very long. So one can stay hidden, but for how long is a real concern among troops wishing to remain unseen.
Before the invention of radars blind soldiers were used to listen for approaching aircraft that were still out of sight.
Aside from satellite, various detection devices and cameras, the methods used to locate an unseen enemy come in small stages of getting to know your enemy. You start to learn certain habits and patterns and then find ways to exploit those.
Skills like tracking, intelligence gathering, laying traps and ambushes all become part of an overall strategy to locate and flush out the enemy. Takes a little time, in some cases a lot of time and patience, especially if the enemy is good at remaining unseen while still engaging on the battlefield, but these are some of the methods put into practice.