I'm getting the below error (from an apex code) that shows a user id that I cannot find a user with that ID "0051p00000AWcQc" how is this possible? 

looked in the user list (non-active also) and ther is no such user in this org. 

Apex script unhandled trigger exception by user/organization: 0051p00000AWcQc/00D24000000JGdl