I have seen door triers in action in James Street some years ago - a young man and a woman pretending to play some kind of chasing game in and out of the doorways - the woman was trying the doors. I didn't understand what I was seeing at the time. The police pointed out that they were opening doors and feeling inside for any key left in the lock. The plan being that possession of the key would enable them to make a burgulary attempt when the household were all asleep. So if a key goes missing folks be aware of this danger.