Doctorlit beat me for the traditional list of errors, but there is still some krautpicking left.

The navigation system should say "Biegen Sie am Ende der Straße links ab."
to turn (left or right) = abbiegen
biegen (without the suffix "ab" that becomes detached for grammatical reasons I can’t explain) = to bend (something)

To get into the story again, I had to reread the previous chapters, and apparently I didn’t take notes when I read chapter three for the first time, so you get this list now:

...the communication link came on before she could open mouth.
Should probably be "open her mouth".

Nikki took the other pistol out of his hammerspace...
Shouldn’t this be "her hammerspace"?

Homura’s attention, though, was caught by the flash patches on his left sleeve.
Is there more than one flash patch on Sergio’s sleeve or are you talking about the flash patches on Sergio and Nikki’s sleeves?

I think that whoever wrote this did actually wanted to say something else...
Should either be "did actually want" or "actually wanted".

I’m looking forward to chapter five-

HG

I was getting worried you had lost interest, and it would be a shame to see one of the most unique PPC stories ever written go unfinished!

Ahem. Fanboying aside,

Interesting name-dropping of N.A.S.T.Y. I've thought about involving them in more recent stuff from time to time, but somehow, I didn't make the connection to the canon kidnappings in "Blank Sprite" and the "Abduction" part of their acronym. Although, now it sounds like that might have been a red herring, but it's still an interesting idea that the PPC's antagonists might not be the typical Suvian/fanbrat numbers for once.

I liked the build-up to the reveal about Agent Sergio's past. (Although you fooled me for a few lines; I thought it was going to turn out to be Sergio himself who owned that bullet, and that time travel was going on.) It's a good addition to what we learned about him in past parts, and I'm looking forward to hearing about his past in the next installment.

And now for the traditional Hieronydoctorlit list of errors:

“And?” Homura asked while impatiently shifting her weight from one feet to the other.

The trio entered the building carefully, looking for any threat. Inside there were a couple of old tractors and an harvester, alongside . . .

“But It might be part of it. Nikki, cover the door . . ."

Sakura looked down in though for a bit, and then nodded. “You’re right. Everything will be alright.”

Those were composed by the half-demolished stone tower Sergio has spotted from the shed, and what remained of a low stone wall.

She didn’t know what to do, and was quite quite surprised when she saw Sergio kneeling down and hugging Homura tight.

It's great to see the story continue! I love your agents, so it's nice to see them again.

Personally, I think the agents are overlooking the obvious explanation - Sakura, Madoka, and Nanoha might not be popular fangirl targets, but they do have a lot of fanboys. But I have a feeling that the truth is something more complicated.