I never posted here when I first saw this thread because I was afraid that once someone from the crew sees this, they might cross it out from their list, but I realized that it's not the case all the time.
I was thinking music. Songs played in tasks or route marker, such as the guitar player is S22.1 Bora Bora or the music at the bamboo dance in S22.5 in Vietnam.
I have a similar thought, musical instruments.
Let's say on one leg a cluegiver is playing a musical instrument, then on another leg it's the background music of a cluebox location, then maybe it could also be at an ARI or roadblock location, or even the pit stop greeter can play a musical instrument.
I think they can be hidden properly to the point that the racers won't notice it and that they won't be able to take note of it.