More than likely you did not encounter an aimbot, but rather a modified set of maps. Aimbots require a modded console with a sideloading piece of code as you all know. The secondary method to cheating and giving the appearance of an aimbot, is to modify global and weapon data inside the map iteself. Since Reach depends on dedicated hosting being passed off to a player, it is possible to have a large group of friends each containing the same modified map. hence, when you go into play, and one of those chaps has a modified map, anyone with the shared modified data, will be prevy to it's effects, or anyone who is designated as such. To explain further...here you go.
Map: Sword base.
Mod: In the weapon tags, each weapon is modified to have an absolute magnatism range of the whole length of map. Anywhere the player looks towards an enemy, no matter how far away, the reticule will move in and seek that player. Next, they modify the magnetism ranges in the globals.global tag, to have an instant magnetic effect...in other words, it doesn't gravitate towards the player...it directly snaps to them. Next, they modify the firing error angles (bloom) to be an absolute of zero, and do this again in the globals.globals tag. This gives a player the presence of an aimbot, but is simply a map modification, nothing more. The effects are that of an aimbot but better since the player does not have a sideloading code that has to "seek" anything.
Furthermore, because of the way the game is different than any other, you can actually specify in the globals, whom the effects pertain to based on team designation. So even if you had the map mod....you would have to be part of their team, to get the effects.
Unfortunatly, Live only seeks out map size and not individual tag checking. Changing values in a mpa in what we call "float" fields, does not increase or decrease the map size. Live servers only check overall map size and map signature (check-sum) before declaring a legit map or not.
In the end, I am sorry that my overly smart modding companions have caused such trouble in the game for you. not all us modders do this kind of thing, as generally we do it for research and to see how things work. I guess it is only a matter of time before ******-bags such as this ruins everyone fun....