you didnt get guard whacked did you? thats the only time ive seen it... or if you steal from them... even after youve been resurected they still show up as grey.... at least thats how it used to be.... havent tried that since the npcs went yellow(i miss farming black sandals from the npc mages)

now that i think about it, if they were on the other side of a server line, sometimes that shows up as grey too....
hope that helps