I have a newbish question about the Covenant's Luminary devices that are used for locating Forerunner artifacts/Forerunner descendants/Reclaimers/humans/the Marathon Man/my gay cousin/etc.

First off, I'll admit I didn't read all of Contact Harvest. I had a lot of things on my plate when it came out and quite frankly, it sucked. Terrible read. Anyways, I just now formed a hypothesis about a potential major plot hole in the whole HALOverse. It's all based around well-established scientific principles, too. So...

Hypothesis:  The Covenant, or at least the Covenant leadership, knew of Earth and all its inhabitants and thus its relevance to their Great Journey long before the events of HALO 2, the video game, when they showed up to Earth seemingly unaware that we were there first, and ultimately long before the events of Harvest took place
Supporting argument/evidence: 

-  The Covenant employ a Forerunner or Forerunner-derived piece of technological gadgetry called, The Luminary. There are many Luminaries in the Covenant inventory, and for all we know there could be hundreds of thousands in existence during the time of the late Covenant empire. This means you have all these Covenant vessels dutifully scurrying about the galaxy, all with a Luminary, by order of the Prophets' decree. Keep that in mind throughout this post.

-  We know that the speed of light is approximately 186,000 miles per second in a vacuum (no resistance). I'm too lazy to look that up in scientific notation or in metric format, sorry.

-  We know upon the most recent and best estimates with modern day measurements that the Milky Way galaxy is in the neighborhood of 100,000 light-years in diameter with a 1,000 light-year thickness. Therefore, it can be deduced that it takes 100,000 years +/- some years for a photon to reach from one side of the galaxy to the other provided it does not become abosrbed by some medium or whatever else Mother Nature can conjure in her infinite abilities.

-  We know from HALO canon that the HALO array was fired approximately 100,000 years ago or slightly longer in years relative to our calendar (we assume).

Those were the facts. Now, here are some basic assumptions derived from common sense/educated guesses (this is the only weak part of the theory)

-  The Luminary must employ some kind of scanning ability to find its 'artifcats' based on the exploitation of the Electromagnetic Spectrum. Think of the EM Spectrum as the blueprint for the universe. Everything in existence constantly radiates and absorbs electromagnetic waves. Planets, insects, you, me, atoms, stars, galaxies, everything. Based on this underlying truth, how else would the Luminary be able to receive and process the information it gathers about its 'relics'? Forerunner Black Magic?

-  Based off the point above, the Luminary most likely (and prosaicly) gets its information from cosmic radio waves beamed in all directions from their origins. The Luminary has an aperture of some sort and is able to triangulate their locations as it receives them, though obviously not in real-time over vast distances.

Based on all these facts and conclusions, it seems undeniable to me that the Luminary (all Luminaries) should've picked up on the relicy information of the entire Milky Way the moment it (they) were activated/used.

Maybe someone knows something I don't know or that I have overlooked. I'd appreciate any insight into the matter because it's something that has bugged me ever since I started thinking about it 30 minutes ago. Thanks.

But you still have to take into consideration that when the flood arrived the forerunners grew past war. That is why when the flood arrived they had to resort to old script about war machines in order to build some type of protection devices or weapons to fight off the flood (even though it was futile) but none the less it was a desperate last stand for them when they built the ark an rings. However I do agree with you Totemo that if they were masters at everything they should've built the halo's to destroy the flood not the food supply.

Here's my sidestepping excuse to end all of this.
The Oort cloud did it. It obscured our signals and shielded us from the Covenant for decades, possibly centuries. I blame the Oort cloud.

who cares the game was an first person shooter game not a thinking game you know boom boom boom game plus the book is different the the game read them all like the flood you don't get to go to alpha base in the game for xbox or the elite zukam isn't in a shade in the elevator but later on in the game he's there and not in another elevator