I was taking a look at my AP5 today.
I havent looked at it since my last range session 2 weeks ago.
I fired ~100rds with no issues.
Today when I was inspecting it I heard something rattling inside.

It dropped out when I pulled the bolt assemble.
Its about the diameter of a lighter flint.
About half as long as a new flint.
Solid metal.  Looks black and one end is silver.  

Anyone have an idea where it came from or broke off of?

Pic from Reddit thread posted.

A picture would probably go a long way in this situation.

Don't forget the banana for scale.

Found this on Reddit, same issue it appears

Reddit Link

Yep, thats it.
Thanks for finding that.  
Not sure what to do about it yet.

So I took out my trigger pack and yep that pin has snapped on both sides.
Weird .
Im gonna order a HK one and a spare.  Not worth contacting Century.

They will usually run just fine without a hammer cross pin, especially on semi auto only firearms. You can still use the "pinned style" hammer and shank, just without the cross pin.

The hammer cross pin is present on 9mm and 5.56mm H&K weapons, as those have a somewhat rare tendency of the hammer shank separating from the hammer during firing. The weaker the hammer spring is, either from repeated use or just because it is a spring made for a smaller caliber weapon, the more frequently this happens.

If your 9mm will run with a 7.62mm G3 hammer spring (many will), then you can likely run it without a hammer cross pin, with 100% hammer/shank function.

The alternative is to purchase another 9mm "XX" marked, pinned hammer/shank assembly, which will usually run about $125 to $150. Even then, you'd need to grind the auto trip notch off of the hammer, or it won't work in an MKE trigger pack.

I successfully repaired the broken pin today.
Ordered the hk pin.
Getting out the old one bent my nice tektron punch. dumbass.
I ended up using a tapered nail set and that got it moving.

Using a estwing framing hammer with the hammer on the concrete.
Its back together and we'll see how long it lasts.
I bought a spare in case this one fails.

I'm surprised that you were able to find one of those pins. I thought they were pretty much unobtainium these days.

Congratulations on not only fixing it, but doing it the correct way.