Purposely breaking patch fiber for a training lab

I'm working on a barebones link troubleshooting training lab and I want to take a patch fiber and break it in a way that's not visually obvious.

The goal of the labs are to really explain things very simply. Tx/Rx and how light circuits work vs. electrical circuits, which most people are quite familiar with. Focusing more on the troubleshooting logic, assumptions that can/cannot be made, etc.

The people I'll be working with are generally fairly bright, so I need to make them actually troubleshoot instead of going "the one with the heat shrink on it is bad" or whatever. Basically eliminating all context that could lead to them to 'cheat'.

For the labs we're simulating light with VFLs instead of using actual optics, for reasons of cost, mobility, and safety, so hoping to figure out way to stop the light completely as we're not going to get into things like 'bad' light, etc.

Thought about moving the strain relief and cutting there, but as we'll be plugging/unplugging a lot as the lab moves forward, I don't want to be pulling one out and have the entire fiber come out, making that cable a dead giveaway, either. I've also thought about just taking a Sharpie to the end, but I'm not sure if that will just come off and/or maybe foul the couplers/coupled fiber when it's plugged in. It might also be a giveaway if the person sees the end. I basically want them to use their IR Card (I will never train someone to look into fiber) and see if there's any light or not.

Anyone have experience in fiber sabotage?