i always worried about leaving my case unlocked but i have flown from and to chicago quite a few times and only once was my bag checked. there was a slip inside telling me they had looked in it but it was dead weird cos nothing was out of place, everything exactly how i packed it, very odd. the time they checked it was when i took some candy bones in plastic jars over for halloween to some friends kids in the uk. im gathering that they may have thought they were real bones!!
my partner has a tsa luggage strap and says it does the job. as for things being planted in your case, a good point and how could you disprove it was you, the only thing i can think of is that it wouldnt have your finger prints on it, scent yes as it would be next to your own stuff in the case. think this is a long shot to happen, wouldnt worry about it londonlad888.