It doesn't have to be China or Russia. As others have mentioned, the current political climate in the US is... "weird". At least, as an outsider, I just don't know how else to describe it. It's like watching/listening to gibberish.
So I can imagine American allies recruiting scientists en-mass, to protect themselves from America. The US has currently demonstrated a desire to take over allies completely (Canada, Greenland), and I'm sure few know who may be next. Some scientists may have simply wished to move abroad, and also, have quite valuable skills which are restricted in some way, hence them "disappearing".
Or the scientists and engineers themselves are wanting out of the US and were offered secret offers to "dissapear" and live elsewhere under a new identity
We’ve just looped back to the first idea this brain dump came up with.
>to protect themselves from America.
not necessarily from America. The goal #1 of the US dominated NATO for example was to prevent Germany from getting nuclear weapons in exchange for protection by US. Now with US de-facto withdrawing, Germany would have to quickly get nukes (as well as missiles to carry them) - i don't see other option for Germany here giving the environment in Europe and MidEast. So they would also need such scientists. South Korea, Japan, Australia seem to be in the similar situation too. (and everybody understands that a nuclear weapons program can't be a long multi-year endeavor - somebody will try to stop you - and so it must be very fast once started, and thus you have to have ready-to-use skills and knowledge)
Keeping the FRG from getting nukes wasn't part of NATO strategy. The succinct reason for NATO was to keep the Soviets from marching to the Atlantic. The more pragmatic was expressed as "Keep the Russians out, the Americans in, and the Germans down."
The whole point of NATO was always to present a united front. Even the slightest HINT of doubt could embolden the enemy to test resolve.
And at this point NATO has pretty much collapsed. Trump turned his back on Ukraine and nobody wants to join operation Epstein Fury.
> Germany would have to quickly get nukes
No shit? Why would they have to? Is someone ready to nuke them if it turns out they’re no longer under the U.S. nuclear umbrella, or are they some special snowflakes who should have them while Iran (and most other countries) shouldn’t?