Being active combatants with numerous enemies, they still need to consider operational security.
The Donetsk separatists are more like a political party than a combat force.
I'm not saying IS are the Donetsk separatists, just that they're not hiding in mountain ranges or tunnels and there's never been a question of "where are these guys".
The Donetsk separatists are more like a bunch of advance troops from Russia than a political party.