There are numerous reasons why one might be suspicious of a ticket the passenger is claiming is valid. To list a few,
- it could be damaged,
- it could look like it has been printed on a computer printer,
- it could be defaced or altered,
- it could be via a different route but the passenger claiming they were told the route they are on is valid when purchasing the ticket.
Just because a passenger says something, there is always the possibility they are lying. I had a chap a few weeks ago on a child ticket, said he was 15, had no ID. I "should" have taken his word on it (the customer is always right, etc), but BTP met the train, and although he was young looking, he turned out to be 20. £80 fine.
I wasn't sure, and in a dim light he would have passed for 15. However, I was right not to give him the benefit of the doubt. Most of the time, the hassle involved in sorting a "suspicious" ticket out means that this course of action will only be taken if alarm bells are ringing very loudly!