Radio Signal From Direction of Proxima Centauri

[Bear]

IIRC a fully developed Matrioshka brain would radiate at just barely above CBR temp and so might be very hard to detect.

The same is true of any fully developed Dyson swarm, whether it happens to be a Matrioshka brain or not.  So, yes, granted, once fully developed they could be hard to see. 

Ultimately the size of a Dyson swarm is limited by the Hill Sphere of its star.  So one would have to be fairly dense and fairly warm inside any kind of tight grouping like a cluster, but could be absolutely enormous around a medium-large star in uncrowded intergalactic space.  

I haven't done the calculations before but... hmm.  The sun's Hill Sphere is considered to be about a light year.  That means about twelve and a half square light years of surface area.  The sun produces around 4e26 joules of energy, making 3.2e25 joules per square light year.  A light year is 9.5e12 kilometers making 90.25e24 square kilometers per square light year, so the biggest possible swarm here would have a radiant energy density of 0.35 joules or thereabouts per square kilometer of "surface area".  0r 3.5e-7 joules per square meter.  Or, 3.5e-7 watts per second per square meter. 

So this hypothetical Dyson would be taking in radiation from the local stars and CMB etc, and then re-radiating that same amount of energy plus 3.5e-7 watts per second per square meter.  Which would, in fact, be pretty damn hard to detect.

That ignores the possibility that anyone is running nuclear power plants somewhere inside it, which would raise its heat density considerably.  But if someone wanted to be stealthy, yeah it looks like they could.