Earlier this year, the ACLU won a case regarding the constitutionally of the no-fly list[1]. Basically, their argument was that you can't give up your Fifth Amendment rights (due process) when you travel internationally. Since this argument won, it implies that Fourth Amendment rights (probable cause searches) also cannot be forfeited.
Relevant quote:
"Accordingly, the Court concludes on this record that Plaintiffs have a constitutionally-protected liberty interest in traveling internationally by air, which is affected by being placed on the No Fly List." [2]
The Fourth Amendment only protects against unreasonable searches without probable cause. The Supreme Court could well decide that airport scanners are reasonable and therefore do not require probable cause.
Relevant quote:
"Accordingly, the Court concludes on this record that Plaintiffs have a constitutionally-protected liberty interest in traveling internationally by air, which is affected by being placed on the No Fly List." [2]
[1] - https://www.aclu.org/national-security/latif-et-al-v-holder-...
[2] - https://www.aclu.org/sites/default/files/assets/latif_v_hold...