I've not seen this before with a multimeter holster but regularly see 'plugpack' or 'brick' power supplies with deep furrows melted into their cases. The furrows are caused when people wrap the power cable around the unit for storage. The insulation of the cable reacts with the ABS plastic of power supply case and melts itself into the surface. It's an odd phenomenon and seems worse when units have been stored in warm environments like lofts. Not all plastic/cable combinations suffer this effect. A more severe reaction often occurs between cable insulation and polystyrene packing material. I have found cables covered in melted polystyrene after having been in direct contact with the packing for long periods of time.
As has been suggested, this is some sort of chemical reaction between dissimilar materials and their respective chemical components and excretions. It can really ruin your day to find that your carefully stored piece of plastic cased equipment has been 'etched' by contact with it's own mains cable..... been there, seen that
![Cry :'(](https://www.eevblog.com/forum/Smileys/default/cry.gif)
Many years ago I was told that a new cars plastic parts excrete solvent vapours for some months after manufacture. I presume these are part of the 'new car' smell that some are lucky enough to experience
In your case, there appears to have been a really severe reaction in the period between manufacture and your purchase. A possible manufacturing error in the holster material mix maybe ? Thanks for sharing this though..... it highlights the issue of damage due to disimilar plastics etc.