When the compressor has been running for awhile, does the metal skin feel warm where you want to drill holes? If so, that's the condenser loop you're feeling. And it is in fact pressed up against the exterior skin, so your 1/4" screw could easily puncture the tubing.
You could try making up a thin slurry of baking soda and vodka, paint it on the skin in the general area you'd like to install your mounting screws, then force the compressor to stay on for a good long time while you observe where the mix dries quickest to get a good idea where the lines cross...
Cheers!