I have so far thoroughly explored Halo: Combat Revolved and found what-I-think-are the majority of it's secret weapons or caches, including:

• Who I think is a Tarkov (ODST) reference (Didn't manage to acquire their Shotgun if you could during my v2.9.2 playthrough, but whether I was actually able to then or simply missed it's trick, you should be able to later into the present version of the mod)
  
• "Oribe Water Pistol" in 'The Pillar of Autumn', 'Assault on the Control Room', 'Two Betrayals' & others I might be forgetting
  
• "Nornfang" in 'The Pillar of Autumn' & 'Halo'
  
• "E3-2000 VKMT Rifle" in 'Halo' & 'Assault on the Control Room'
  
• "Markel's Shredder" in 'The Silent Cartographer' (Psst: The Security Station)
  
• "Dupon's Dragon Breath" in 'The Silent Cartographer', 'Assault on the Control Room' & another level (If it wasn't '343 Guilty Spark', then I forget which)
  
• "Halo 3 Assault Rifle" in 'Assault on the Control Room'
  
• The Covenant Weapon Caches in 'Assault on the Control Room', '343 Guilty Spark', 'Two Betrayals' and others (Which include other weapons such as "Fury" and the Shredder variants)
  
• "Vengeful Spirit" in '343 Guilty Spark'
  
• The Third "<UNREGISTERED WEAPON>" in 'The Library'
  
• "Tank Girl's Temper" in Two Betrayals
  
• "Daylight's End" in 'Keyes'
  
• "Rocket Jumper" in 'The Maw'

As well as the Secret Room in 'Two Betrayals' and the Phantom's wreckage & Overshield in '343 Guilty Spark'.

But, for the life of me, I cannot seem to find the "XBR55 Prototype" in at least 'Assault on the Control Room'. I have scoured the entire level to the best of my ability, even in a Banshee I got lucky enough to shoot down intact during the Fireteam Zulu sections on various difficulties and yet I cannot seem to find this weapon in the present version of Halo: Combat Revolved. Has anybody else had any joy in finding this weapon? I'm really hoping it wasn't removed, as we've lost enough of Halo: CE+ as it is! ([Me from the Future]: It returned)

I know it existed in Halo: Combat Revolved at one point in time as it can be seen here (Although the "Halo 3 Assault Rifle" seen here has since been swapped with a rare Plasma Rifle variant):
https://www.youtube.com/watch?v=qCKqglbIi2s