Personally:
Enyo Island, Medusa's Head (52066, 70305)
Argonaut Hunter L17 (6/26/14) 26.36 Ped, and Argonaut Scavenger L13 (7/19/14) 24.55 Ped
Disclaimer: I own Enyo Island and do all my Argonaut grinding there, and I have killed thousands of Argonauts!
Soc Mates have looted several on Island as well... Mostly from Guardians and almost all were in the 25-35 Ped range. A few have dropped during Events; no details other than woots in event chat. All and all, I have seen maybe 10 ESI's drop on Enyo Island since January. Hope that gives you some useful information.
Chad Sandman Dreamweaver