I'm surprised that your shop steward didn't know how to properly handle the situation.
First, it clearly states that management cannot use technology to discipline anyone unless it's related to dishonesty (A. 6, S. 6 of NMA).
A bargaining unit employee must be "caught in the act" as they say when it comes to seat belts or bulkhead door incidents. Telematics alone is not enough.
Second, we have progressive discipline at this outfit. You cannot go directly to a warning letter. There needs to be a TLA (a sit down) before warning letters are handed out.
Write a rebuttal letter (your steward should have these), and just use a little more caution from now on.