So I finally was able to contact Air Venturi. They wanted an original sales slip to initiate a repair. So I did not have that handy, so I thought it would be a good idea to disassemble, and see what I could see.
The air rushing out of the barrel was due to the rifle not being cocked on filling. So that was not a "real" issue or indicator of the leak.
Air still rushed out of a relief hole at the receiver side of the gas cylinder below the wood line ahead of the pressure gauge. So I removed the cylinder and inspected the two 2.5X23 Buna 90 orings near the relief hole. They looked good with no signs of wear or cracking. I replaced them anyway with smidge smaller 2.5X22 Buna 70 orings that I had in one of my oring kits.
(I did this mainly because another member shared his S510XS leak resolution:
https://www.airgunnation.com/threads/looking-for-help-with-a-leaking-aa-s510xs.1305340/ )
I put a super light film of silicon grease on the orings, and cylinder thread. That allowed the cylinder to tighten a lot easier than when I loosened it.
As a buy one, get one free fix, I had a foster fill conversion that I installed at the same time. I filled the cylinder to 100 bar and waited a couple hours, and saw zero evidence of leaks. So I filled to 200 bar and let it sit over night. Still zero leaks, and held the pressure on the gauge.
So according to my research on the forum, this is at least the 3rd issue with these same orings on a newer XS. I'll wildly guess AA sourced some marginal orings. Amazon lists the 2.5X23 Buna 90, but they are out of stock right now. I have twenty of the nominal smidge smaller buna 70, so I don't feel compelled to get the "spec" oring at this time. I'll look around a bit, and maybe get the spec orings at a hydraulic shop if I get up that way.
Anyway I am real happy that I solved it, for no cost, no shipping time and shipping risk. Thanks to those that shared their experience with their S510s.
