I just got one of these in a derelict tower mission on deimosI tend to almost always run into at least one of em doing derelict runsThey dont drop at all lmao killed about 150 of these guards 0 in my pocketRNG be RNG. I got 4 in as many runs this week *shrug*Your not killing the right thing, they Always drop... like it saysPlanned for 7 of these to be a bile feed.
Its a miniature part of the grineer forest tileset, often appears near a toxin injector as a big rotating wall opposite a tunnel/dead end.
so now i have a part for a weapon that is 3 mastery ranks away
that makes no sence
Just wait until you find out about prime parts lolIt'd be nice if we could use these for literally anything else other than selling fodder. 
Of course, when I sell my stockpile of hundreds of excess capacitors I didnt realize I had, they'll release a new weapon requiring 200x of them. :(
Dropped from Prossecutor with 100% rate on Lua > Tycho during Sortie second mission.
Took me about 12 runs on  Rescue mission to get the 6 pieces. If the prosecutor does not spawn when you hack the jail entrance, restart. Or finish the mission to collect a bunch of spectres
Just did Bode on Ceres (Spy Mission).  Dropped by Prosecutors. Ran into one while running around. 
During extraction, was notified more Prosecutors were now approaching.  Went for extraction as quick as I could, but ran into another Prosecutor on my way to Extraction.  So, they will appear here.  Good luck!