#Muriel spend thousands of years finding stuff out but their memory got wiped because it could have ruined the great plan

[insanity-laughs-under-pressure]

Theory that Muriel did some investigating of their own after Aziraphale questioned them on the whole Job situation, only Muriel found out too much and had their memory wiped. It would also help the Metatron to leave them on Earth in the bookshop because it means they're out of the way from discovering things again.