I do school grounds and sports pitches, where i used to get a lot pinched, obviously they saw the markers. Now i limit the school fields to setting 5 traps on friday, checking saturday and collecting sunday. I dont mark them either, i just cut out the sod and sit it next to the trap, you can still see it easy enough because you know what your looking for and roughly where, but the potential thief cant see any markers so ignores the area.
On sports pitches i sometimes use sheep marker spray, this is good as you can see the traps when on top of them but from a distance again , no marker standin