It doesn't effect the IQ nor does it show up at all by shooting a longer exposure as a dust mark.
You pretty much answered yourself right there bro. To continue to worry about it is ludicrous.
Although, I suppose, it also depends on what you're using the lens for. Did you pay $2500 to use the lens to photograph stuff? If so, shutup and go shoot since it doesn't affect quality. Did you pay $2500 to look at the internal mechanisms and expect perfection? If so, then you have a problem, and should return it.
My final words in closing: be logical about this.
I dont think there is any need for your tone tbh.
Ive had a 70-200mm L F2.8 for 5 years and its had a few owners before me, its nearly 20 years old and had nothing in it not a spec. To spend nearly £2k and its not $2500 its $3000 equivalent here in the UK and have a rather large piece of what looks like dust but like said looks like a spec of metal. If that is acceptable to you then…
I am being logical about it. Which is why I asked the forum opinion. I am a professional photographer and have never had an issue with any of my L lenses coming from the factory with any sort of large particles already present, after years of use you do expect it then you have them serviced, but not straight out of the box
The reason for asking is I think it may have had a bit of a bang in transport and don't really want to be a year or so down the line out of warranty and have issues with it. I don't think that is unreasonable. If it is serviced and they still can't sort it the likelihood is that they won't take it back with it being tampered with.
If I come to sell it and being honest in my description I say there is a large particle toward the rear of the lens, (with Zoom lenses particles are accentuated toward the rear) most people would browse over it or want it at a good deal less than if it was in better condition. To start off with a good version I don't think is unreasonable, especially with how much the lens retails.
Thank you everyone for your opinions and help