enyoueffsea
On Moderation
Not if you're asking a passenger who (I assume - perhaps the OP could confirm) already told the RPI that they had joined the train at Luton, whether they joined the train at Luton.
Looking at the letter, however, it looks like they're not even doing that.
They're just asserting the ticket wasn't valid when it was, based on non-existent rules they've concocted about tapping in/out requirements (a failing for which they have form, as I can personally attest) and asking for comment. That's not a legitimate line of investigation. It's completely misconceived. The ticket isn't rendered invalid just because the RPI couldn't see evidence of a scan at the origin. It's only invalid if the passenger was short-faring, but they haven't even put that specific possibility to the OP and asked for comment.
How many times do you think RPIs hear “I did scan my ticket, honest, it must not have recorded”? The RPI will have no way of checking or verifying that on the spot and therefore they were right to follow their process to submit for investigation.
The TOC has not at any stage set any rules on tickets being scanned. The TOC have sent out their standard letter asking for the OP’s version of events. Subject to the OP providing the information and evidence they have suggested they hold here, that is likely to be the end of the matter.
It’s an unfortunate series of events with the OP holding a well known short ticket which did not register a scan on the day the TOC held an operation of checking all tickets.