Sounds like a crime of opportunity more than a collector grabbing a valuable card to profit. The way you said 'in a display cabinet' makes me think that the person may not have known their value, but having them in a cabinet would let them know that there is some sort of value to them. This probably means that they will get sold off for bugger all as they would need to tell the buyer that they are hot if they want to keep it on the DL.
Punks!!
No one would think that they need to lock up their possessions in their own house though!
Hope you find them mate! I will keep an eye out, although tbh, I usually don't pay attention to anything Rondo for sale! lol
BTW, if you want help/ideas to lock up the cabinets you use, I can help