help me identify this part after replacing cam follower

[Doomhand]

I recently bought a gti mk5 with 170k miles and I'm currently doing some maintenance. My cam follower was very worn so I decided to replace it before any failures. I removed the hpfp and I had a feeling something popped out of place and fell somewhere. I didn't pay much attention to it and put everything back together. I went for a ride and everything was okay and car went into boost juse fine. Today I popped my hood and i found a little metal piece that looked like a tiny battery sitting on my transmission. I'm wondering if its part of the hpfp and if I should try to put it back on. I'll attach some images.

[OllieVRS]

I've taken off the HPFP a few times, but never disassembled it. Never seen anything like that.

But it would be weird that something round like that could just 'pop out' of an pretty tightly assembled device. Do you know if your HPFP has been upgraded by a previous owner?

There's a old post on here showing the complete disassembly: https://www.mk5golfgti.co.uk/forum/index.php?topic=10965.0, but due to its age the photos are not so clear.

Maybe someone else who's done the HPFP internals upgrade could chime in, as unfortunately I can't help any further.

If this was my car I'd be panicking, as I've found various strange foreign objects in my car's sump, like electrical wire, orange plastic (turned out to be part of cam chain tensioner 3 years later) and metal shards (origin still unknown) and my car has ran mostly fine. So if you're boosting fine I would assume it's most likely okay. But no guarantees 

[Doomhand]

Hey man, thank you very much for your reply!

I found out it was the HPFP retainer cap. I dissambled the pump and put the little cap back in place. Everything seems okay so far.

Here's a video that describes de full disassembly. Its really helpful.

You are right, these cars seem to have weird bits and pieces laying around sometimes 

[OllieVRS]

Glad you found it!

I watched the video you posted but I'm still not sure which part in the video is the one that fell out of yours. The retainer piece looks like a different shape to the simple flat metal cylinder on your photos earlier.

By any chance did you take a photo of the reassembly of the pump with the piece visible?

Very curious in case something like this ever happens on mine.

[Doomhand]

Glad you found it!

I watched the video you posted but I'm still not sure which part in the video is the one that fell out of yours. The retainer piece looks like a different shape to the simple flat metal cylinder on your photos earlier.

By any chance did you take a photo of the reassembly of the pump with the piece visible?

Very curious in case something like this ever happens on mine.

Sadly I did not take a picture but check the video that I uploaded. It has all the info you need.

I can see from the post you linked the little retainer as well, the one that you actually hammer down to put back into place.

[pudding]

That is the little button that sits at the end of the HPFP piston. It has to go the right way round otherwise the follower won't spin.  One side is flat and the other side has a raised bit in the middle.  The raised bit is what must contact the follower.

The fact it fell out of the HPPF is not a good sign at all.