Speaking from the flip side of this.

Personally if someone gets on with a pass I watch the ticket machine for obvious discrepancies ie A 20odd year old with a Pension Pass because the ticket machine tell us the difference. Anyone and I mean anyone Age, Race or Otherwise gets on with a disabled pass I don't even bat an eyelid. As people have already pointed out not all disabilities are visible and some people don't like talking about them.

As for C+ Cards. When outside Teesside and North Yorkshire when a C+ Pass is placed on a reader it comes up on the screen
Concession Only
Concession + Companion
If the driver is being a grump the most he has to say to you is "is it just you travelling" so he can issue the correct ticket.
Honestly it boils my blood when people make trouble for themselves because they are bored. Next time you get chew from a driver make sure you keep your ticket and stick your boot in!