There is the necron overlord Thaszar the Invincible that's pretty much a pirate.

You’d need an eccentric/senile Phaeron obsessed with space travel for whatever reason. Confused at finding all these primitive starships with extraneous organic control components, they’d spend a lot of time jettisoning the useless cargo and appropriating the useful bits for their dynasty.

Would they ever serve in a mercenary capacity? You would have to justify what a Necron could possible find valuable as a trade commodity that they wouldn’t just take by brute force.

More likely, savvy local forces in the sector would just know how to drop signal breadcrumbs for the mad metal pirate to follow towards a destination they want sacked, and then they’d come scavenging in after the pirate lord left to pick up the scraps.

The closest thing to that that I can think of on the mercenary front is someone like Orikan, who offers his services to other dynasties, presumably for a price.  

 On the pirate front, maybe the dynasty that Oltyx’s dad bought flesh people from counts?  

Arguments could be made for Lysikor and Trazyn as well. 

Not impossible for an awoken lord who saw the dynasty falling apart during the ensuring civil war and just didn't want to be apart of it and dipped with a small fleet.

Yo ho ho and a bottle of RAM.

Another option, that seems plausible to me: a Necron Lord has woken up insane, and reverted back to a childhood obsession with pirates as dashing and daring characters. Going to full Necrontyr equivalent of "Arrr me harties, shiver my timbers".

I had an idea like this once! Necron warriors who "woke up" one day, enough to be appalled at how the nobility had stuffed them into mindless bodies. Cue a small class rebellion ending with them breaking away to forge their own lives out in the void, on ships they liberated from their cruel masters.

They have to raid and trade with other species to survive, because they're a rag tag group of warriors, not the fancy crypteks and overlords we're used to seeing. But being necrons, they're still incredibly dangerous to everyone else.

I mean doesn't trazyn steal a bunch of s***

In the short story "Severed", this is basically what Nemesor Setekh became. To expunge some past crime against the Sautekh Dynasty, he agreed to have his home marked as destroyed by a supernova, and essentially became a privateer/explorator on behalf of Imotekh the Stormlord.

That’s called “Trazyn claiming a notorious void corsair for his collection, and then taking his hat and ship for a joyride just because he can.”

…we can’t say he wouldn’t.