Hmmm, I didn't think there was a way to do that...
Unless they took one of the snipers already in the game and put them in a custom group? Or made them a boss objective and re-named them?
I don't think any custom critter would be a true sniper.
You cannot create a truly custom, true sniper at this time. You can reuse existing snipers with some care, and you can create a custom mob that is somewhat better at spotting things than typical by giving it a Primary or Secondary with a toggle Perception power that they can use.
<font class="small">Code:[/color]<hr /><pre>
Range at which they can see you
Type Base Stealth Invis Smoke
==================================
Minion 45 10 x 4.5
Per. Min. 72 38 18 24.3
Lt. 50 15 x 5.0
Per. Lt. 75 40 20 27.0
Sniper 149 114 94 ?
</pre><hr />
As can be seen, a custom Lt. with a Perception power such as Targeting Drone is enough better than a standard enemy that it may give you the effect you want, but is still inferior to a real Sniper. (Table was constructed by cross-referencing various wiki and Red Tomax pages, so if there's someone who has corrections, please post & explain.)
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Interesting, Miuramir, thanks.
For the record, my first draft of the unit I had in mind actually was /Devices with Targeting Drone. Thinking of Snipers, I made this unit a Lieutenant, but found that a Lt couldn't have Targeting Drone without also having both Smoke Grenade and Caltrops, two of the most annoying enemy powers in the game.
I was testing just now, and discovered that a minion can have Targeting Drone, and will receive Caltrops but not Smoke Grenade. This revelation is terrible since I just published my epic arc a few hours ago .. but now I've taken it down again with an eye towards a possible redesign. Thanks a lot! 
What other custom power sets have +perception? Maybe I won't have to do as big an overhaul as I'm worried I might.
SR gets it (as does /nin, but it's...you know...busted). I WP gets it with heightened senses. An AR/SR or WP could work as a stealth spotter without the annoyances of /devices.
And I know that custom critters will use snipe if they get it. Not sure if AR Lts get it on hard or extreme though.
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I saw a custom sniper in an MA arc once. How can I do this?
Thanks in advance, Z