How do you know when a soft sensor / inferential model has silently gone wrong?

Working on a research project (embedded ML + reliability) looking at inferential/soft sensors in process plants (distillation, reactors, etc).

Curious about real practice: when operating conditions drift from what the soft sensor was tuned on, how do you find out? Scheduled recalibration only, redundant hardware check, or something else? And when it does go unnoticed for a while, what does that actually cost (bad control moves, off-spec product, alarms)?

Not selling anything, just trying to understand if this is a real daily pain or something plants have already solved well enough. Any war stories welcome.