<PERSON>'s answer - Hidden in Plain Sight - is excellent.
I'm just going to develop that idea a bit further, in a way that's too long for a comment.
<PERSON>' answer boils down to: don't hide it - show it, but as an either utterly boring or unattractive phenomenon. With several good ideas on how to do this. I'm going to focus on the boring, "can't be bothered" aspect, inspired by two authors.
1. <PERSON> (HitchHiker's Guide). Making anything invisible is incredibly hard. It's much easier to surround it with a Somebody Else's Problem (SEP) field. People observe the thing, but instantly turn away and forget about it.
2. 627
<PERSON> idea of "interstitiality". In that novel <PERSON>'s protagonist is a member of a quirky cult, one of whose rules is that, when forced to enter the normal, "unclean" world, you must never do things in the usual way, but find a way to slip through the cracks between systems, customs, normal way of living. (The Hungarian concept of the kiskapú or "small [side] gate" is similar, but refers to getting things done in an overly bureaucratic state).
What I'm suggesting is to engineer the invisibility of your planetoid, but using social rather than technical engineering.
How, in the context of your world, might you encourage anyone who might be interested, who happens to stumble across evidence of your planetoid, to actively not want to investigate any further - to react not with curiosity but with indifference, or even with an active desire to do anything else (clean the bathroom, de-claw the cat)? In other words, how to create a real SEP field?
Perhaps the disguise this planetoid adopts, in official records, could connect it with a difficult, burning issue, which people would rather just not deal with. A recent traumatic war; a pandemic; some religious prohibition. Whatever impression your planetoid projects, it needs to be tiresome rather than challenging, evidently boring rather than noteworthy or enigmatic.
Or, using the idea of interstitiality: you disguise it (using an artificially-created emissions spectrum, for example) as one of millions of objects which fall between two jurisdictions. For example: according to Galactic Exploration Institute's records (which you have "improved" for your own purposes), this object is being surveyed by the Institute for Galactic Exploration; according to IGE's records, it's the GEI who are responsible for investigating it. Both the GEI and IGE are immensely bureaucratic, and they loathe each other. 239