Been into hacking this one lately, and stumbled onto something that nobody seems to have acknowledged before. Basically it's an alternate version of the mission "Precious Cargo" where you ride a tank. More details are in the description of the video for anyone that's interested, but this is an entirely separate mission file leftover on the Gamecube disc. Sadly the mission is almost identical otherwise, but still pretty interesting (and fun).
This feels to me like something that would've been accessible via a button code (like the hidden race in 007 Nightfire), but for now I can't really confirm anything about it. Like I mention in the video description, could also be a gold medal reward that went unused.
As always, I like to make codes for other people to check this stuff out:
007 Agent Under Fire famously began life as a remake of Eurocom's 007 The World is Not Enough (N64). The title is also somewhat notorious for a softmod exploit possible with both the PS2