You definitely will get a red beep for a documents check. Using online check in and self printing of boarding passes doesn't stop CX from checking your documents. It's happened to me multiple times.
I did a three part trip last week: CX from HKG to TPE to meet up with mates, then JL TPE to NRT for rugby then back on UO.
I couldn't even get a BP for HKG-TPE one-way on Avios tix. Had to be printed at desk. Docs checked and onward tix asked for.
At TPE, online BP was obtained, but docs check was still carried out on me when one of my mates checked in bags. Strangely, onward tix (which would've been the UO tix) not asked for.