They are idiots arent they....well some of them are....some (which are worse) are actually quite smart....
The logic....nick the face of the stereo, easy peasy no tools needed, come back a week later and nick the nice shiny "new" headunit that now replaces the one with the missing face!...
This logic was commonly used with house burglary's....the gits would rob your house of all the goodies, they return a month or so later to nick all the brand spanking new stuff that the insurance company had replaced the missing stuff with....the first job was almost a neccesary evil on their part to get the greater bounty on the return visit...
And they normally knew when the stuff came by watching for the LCD tv boxes stacked up next to the wheely bin on rubbish day...some of the gits even nick the boxes so they can pack the TV's back up when they nick em again and then pass em off as new for more £££'s....
Believe me, this is quite common, worked delivering electrical goods for years, a fair chunk of it was insurance replacement stuff, and sometimes several times to the same addresses!!!
If it was me I would find out if you can identify the perp from the cctv, and "educate" him all the way to the cash point machine for some compensation...
The now useless headunit could get some razor blades superglued to the back of the casing and then be refitted....at least then if they come back and nick it, and quickly shove it inside their coat out of sight....the thief will be easily indentified as the hoody having multiple stitches in casualty!!! A least there will then be dna evidence a plenty at the scene if the cctv is clear!!!