Assuming this was done at the government's request, I assume Hetzner is more than willing to comply with a court order mandating they allow them to monitor and physically size a machine.
And outside of nation-state requests, even ignoring the fact that someone could probably pay-off an employee, I think ease would depend a lot on the datacenter and target; judging by the awesome and hilarious story behind the Fremont Cabal accidentally becoming an internet exchange by essentially having some dude barely secretly slipping unauthorized cables into the raceways [1], I figure there are a lot of places where if your target is simply renting a couple rack units or single rack rather than an entire locked cage, you can probably get physical access by doing the same.
Also, a lot of Deviant Ollam's stories about industrial security and the dozens of ways he's broken into utility companies, server rooms, etc — mostly just by being confident, looking the part, bad doors [2], and badge cloning [3] — don't give me a ton of confidence that someone with skill couldn't feasibly either get direct access to servers they shouldn't, or at the very least, access to an important part of the supply chain for their target.
And speaking of supply chain, my processor died recently, so I ordered a brand-new in box replacement Ryzen, and when it arrived last night, out of curiosity, I wanted to see if I could get the CPU out of the box without breaking the tamper-evident authenticity seal...
... and about fifteen minutes later, after borrowing a syringe and hypodermic needle from my mom, a little bit of isopropyl alcohol, a blade from my safety razor, and a quick look at a video from LockPickingLawyer [4] and a couple from datagram at DEFCON's Tamper-Evident Village [5][6], I had the CPU out, put my old one for now, and re-applied the sticker with no visible damage to the box or seal.
All I had to do was tip it upside down at about 90-degrees, douse a little bit of the alcohol under the top of the seal, let gravity do most of the work, and then carefully lift the seal with the razor. After that, I just lightly squeezed the box to make the front tab come as forward as possible, and then carefully pushed the ear flaps down to prevent tearing, and then I was in.
I've seen others demonstrate it on older AMD boxes that had flexible cardboard in-place of the cooler, allowing them to pull the cardboard to make enough room for tools to get out the CPU without even touching the seal [7]. But in my case, it was a newer box with hard plastic inside where the cooler would've been, so that's why I went for the seal instead.
No surprise to me now that counterfeiting is rampant on Amazon, with people returning the box after putting in either random junk, dead Athlons covered by a counterfeit serial-matching IHS, or the cheapest socket-compatible CPU after deluding both and swapping the IHS.
I figure with a bit of practice and better tools, like Teflon spudgers and syringes, it'd be significantly easier to get past 99% of tamper-resistant/tamper-evident seals and into boxes you're not supposed to be without risking damage, and then you can intercept a package, compromise something critical, like the server BMC or firmware, reseal everything, and be on your way.
And given the relatively recent scare with loads of servers, including Dell and others, being shipped with "AMERICAN MEGATRANDS" labels on their BMC boards, with no one noticing until a YouTube commenter pointed it out during a teardown by ServeTheHome, I think it's totally feasible for an enemy to just compromise the entire physical supply-chain of a company, datacenter, or whatever else [8].
[1]: Oxide's On the Metal: Kenneth Finnegan - https://oxide.computer/podcasts/on-the-metal/kenneth-finnega...
[2]: Deviant Ollam @ Shakacon: The Search for the Perfect Door - https://www.youtube.com/watch?v=4YYvBLAF4T8
[3]: Deviant Ollam / Modern Rogue: Getting an RFID Implant - https://youtu.be/SZiRISGdQ4g?t=277
[4]: LockPickingLawyer: Did I Cheat On This Challenge? (Tamper-Sealed Abus) - https://www.youtube.com/watch?v=xUJtqvYDnkg
[5]: DEFCON 19: Introduction to Tamper Evident Devices - https://www.youtube.com/watch?v=W07ZpEv9Sog
[6]: DEFCON 30: Tamper Evident Village - https://www.youtube.com/watch?v=slhdowWjSuU
[7]: cycurious: How Counterfeiters replace CPU in Sealed Retail Box - https://www.youtube.com/watch?v=Bni8bgGlXDE
[8]: ServeTheHome: Dude this should NOT be in a Dell Switch… or HPE Supercomputer - https://www.servethehome.com/dude-dell-hpe-ami-american-mega...