I’m not sure if there’s a lore explanation for this, but I couldn’t find anything on Halopedia so I thought I’d ask here. Requiem was the first shield world built by the Forerunners, but for some reason there’s no mention of it ever having a monitor. I know that the Prometheans were already stationed there to guard the Didact, but from what I understand there wasn’t any rivalry between them and the monitors (in the CEA terminals, 343GS says he wishes he had some of them on Alpha Halo). One of the other shield worlds has a monitor (although the one from Halo Wars didn’t, either), but was Requiem missing one for a reason or was it just operating under automated protocol?
Just because we didn’t see a Monitor doesn’t mean one didn’t exist. Look at the Ark, for example. At the time of Halo 3, we didn’t know whether the Ark had a Monitor. It was only after H3 when Hunters in the Dark came out that we learned the Ark did have a Monitor, 000 Tragic Solitude. Requiem’s Monitor could have been off doing other things (or it could have been taken offline at some point, perhaps by the Didact after his release) during the events of H4, which would explain why we didn’t see one (just like we didn’t see Tragic Solitude during H3).
At first I assumed the Librarian’s imprint on Requiem was a monitor of sorts, but I’d go with Chimera30’s explanation above