A Holoecho marker can be targeted by attacks? For example a BS Attack or an Hacking Attack? Or should i before Discover all the fake Holoecho to understand wich is the real one?
You can directly attack them, but you have to choose one of them. Of course, if you shoot a template so you get all of them, you don't have to think much about it.
And you can always "hold" your ARO against Holoechoes. Means you lose it if they don't reveal, but still helps.
In your active turn you move a silly ordermonkey in LoF of the holoechos. ARO? No way i'll reveal myself for this mere lackey! Proceed to remind your opponent that Combi Rifles are B3. Place 1 on each and have fun with the rest of the orders. Not on the monkey, of course...
And if you find yourself in the position where a unit can Discover+shoot on the Holoecho, you can get one extra "burst" worth of holoecho-stripping in.