Hmm this is my personal experience...
PC patrols seem to drop mets rather in a 'random' kill. I've gotten like 3 at least.
GoldGuards: this was rather interesting. I killed a BR, he called a bigger guild, which was enemied to mine, so I had a little shoot the guard users. Like 5 trojans came around to play... All using guards, and meh my friends weren't online so I used my own goldguard to multitask the guard killing. End of the fight was 89 pkps (do your own math how often they got killed) and 3 mets out of like 40+ dead Goldies... I kinda was cool, thnx Imma go TG now

SilverGuards: got a db like 2 years ago... was OMG x,x this was in a house even lol and my hubby was wtf?
Coppers... and irons never got anything...
Town Guards: just lost a shitload of pots tanking them, never got anything but money
Rat (warrior rb summon): just a lot of d/cs hehe
bat (trojan): cash...