I don't know for sure what those bumps we are seeing on various fighters abdomens are but like others have mentioned it could just be lipomas (fatty tissue lump often seen on lower BF individuals). If this is actually scar tissue built up from injections (you do see scar tissue build-up/lipolysis in people who don't rotate their injections sites like some diabetics), I personally wouldn't know why someone would administer PED's via the abdomen. The abdomen is an area for drugs administered subq (like insulin or enoxaparin) whereas PED's are more likely to be injected IM (in the delt, glute or thigh).
These are just the thoughts of a nurse who has experience giving various shots and spends some free time learning about things he can do for performance enhancement to catch up with his fellow Sherdoggers (I'm only 6'4 and 285lbs, much smaller and weaker than my brethren).