when a battle happens in expedition the debris is lost 
why not leave any battle debris at the coordinates of planet/location 10

I do not nowk eather, but recently it happends the same in to my sistem, there is no informacion on procedence of this kind of army´s.

But is important to take in consideration the fleets lost it in batle, because there is no information on what happeded. 

Battle Report6 Mar 06:12:50
Enemy fleets met near coordinates [50:613:10]
Debris: 76 800 metal and 35 700 minerals
Cetron got:
0
Cetron lost 146 points (150 Viking, 1 Fighter, 1 Stealth fighter, 1 Colony ship, 1 Recycler, 7 Espionage probe)
The battle finished with assaulter`s win
After the battle, your heavily damaged fleet (28 Viking, 1 Espionage probe) tries to reach planet Impulse-Solar-IX [50:612:9] for recovery

This is my case, I sent 150 vikings on fleet comander exploring, and they were atacked by unknowk ships. from the 150 only 28 return to planet.    And I am planing 7 exploration merchindise ships.   Hopefully the politics on place will change soon as better.

Always take care on rute.