Just to add my tuppence worth, from a Guard's point of view.
The Transec announcement is not one that I enjoy making, for several reasons. I find the instruction not to leave ANY items unattended, on stations OR ON TRAINS, ridiculous. If your suitcase is in a luggage rack at the end of the coach and you are in your seat 30 feet away, then it is unattended. When the buffet staff then entice you to walk 5 coaches and buy a coffee, we do not seriously expect you to drag your luggage with you and so, again, the luggage is unattended. This is normal practice, and so you do have to wonder what is the purpose of the previous announcement? And one can't help but imagine that anybody in the business of leaving something dodgy on a train is probably going to put it in the rack along with everybody else's suitcases, rather than leave it alarmingly 'unattended' stuffed behind a toilet and ticking loudly. Similarly, what does the average rail passenger, travelling in a packed train with hundreds of other completely random people, deem to be 'suspicious'? Anything sufficiently worrying to spur somebody into reporting it is likely to warrant such a response with or without an announcement to encourage them. Nevertheless, we are told we must do it and so...
My preference for announcements is to offer a welcome aboard, state the service and it's calling points (on leaving major stations or where a large number of people have boarded) and scheduled arrival time at the end point (lots of people ask this, so clearly it is relevant). I will point out where the quiet coaches are and remind people of what this means (we have been told this isn't important, but I disagree having found plenty who were completely unaware of where they were sitting and blissfully annoying everybody else in the coach) and, depending on how keen my buffet staff are, I will advise people of where the refreshments can be found if I don't hear the steward do it.
Minding the gap is something which should be mentioned only at those places where the gap is abnormally large, in which cases it is well worth mentioning. As for leaving stuff behind.... Many people moan about it many times, and I can understand why. However, I find many, many things left behind on my trains, and so I consider that there might be even more if I didn't irritatingly remind people not to forget things every now and again
