Do these suspicions require some sort of preliminary-evidence of those suspicions before acted upon?
IANAL, but my rough impression from the opinion is kinda. It seems like the reasonable suspicion can emerge entirely from the interaction itself, but that it should be based facts/reasons the agent(s) can articulate.
I found the section titled "7. Reasonable Suspicion, not Probable Cause, Applies to Both Such Searches" on pages 33-38 of the actual opinion (embedded on https://www.aclu.org/legal-document/alasaad-v-mcaleenan-opin...) provided good background on this.
The judge is evaluating one element of the relief the plaintiffs sought--that the agencies be required to expunge data copied during a search which, per the ruling, is unconstitutional. More specifically, the judge is addressing one stated reason for seeking that relief--concern that data obtained unconstitutionally will leave them forever flagged for extra scrutiny.
> "in the future, whether information has been retained from prior searches or not, agents must be able to point to specific and articulable facts for reasonable suspicion to believe that Plaintiffs’ electronic devices contain contraband, which also addresses the concern about any likelihood, greater than the general public of U.S. citizens returning to the U.S. borders, of being subject to a non-cursory search."