[Eidolon Sniper]

Spoiler for not everyone has seen all Gundam 00 episodes yet!:

Episode 1 - I think the entire AEU base was already alerted at the coming of the Exia, so it does not say that the GN particles were not at work there. I think GN particles only work when they're NEAR to the source, or else, if they also work far away from the source (nd I think this was already pointed out by someone else on one of the Gundam 00 threads), I am pretty sure the AEU camera wouldn't have caught the Exia intrusion and so there would be no way to determine that an infiltration aLREADY happened and that they wouldn't also know Patrick the ACE was already being attacked as teh only people who could attest to that are the people watching the Enact exercise with no way of communicating to the base that they ARE under attack.

Episode 2 - they were right smack in the middle of the Ceylon war, so there are a lot of other MS already duking it out in the first place. Graham happened to be in the right place at the right time.

Episode 3 - I think Sergei was also alerted that the Exia was near their vicinity and so he managed to get there as well and tried to rip the Exia's head off.

Episode 4 - I think Graham was alerted that the Gundams were in the vicinity and it is very likely that they might end up meeting one (Graham and his 2 other Flag buddies).

When it appears to be that they were able to call a lot of reinforcements in order to destroy Exia, it has always happened in events wherein there are already a couple of spare MS in the battlefield and saw for themselves that Exia was right there with them. The GN Drives seem to work only when the MS pilots themselves get close to one of the Gundams, so when they are still quite far from the Gundams and managed to have a visual of the Gundam, it would make sense that MS who have seen the Gundams would follow them, or if enemy battleships spotted the Gundams prior to their attacks, manage to send reinforcements their way without the pilots requesting for backup themselves.