Sadly, those "water stains" are normal... It's left over flux from the modern no-clean soldering process. Whoever Sophos is using for a PCB manufacturer is clearly doing a poor job, that should have never made it past quality control. If you want to make it look pretty, just take some 90% isopropyl alcohol on a lint free cloth and you can clean it right off. It may correct the 5Ghz issue as well given it's right on the RF traces. Just make sure you let it air dry for about 15 minutes afterwards. As for the oil on the heat sink, it looks like Sophos is using a machine applied thermal paste which is not ideal. They really should be using a thermal pad instead as they don't leach liquids, something that can be huge problem in PCBs that are used vertical (just like a Wifi AP). Again, if you want to clean it up just use some alcohol and remove all traces of the oil and old paste. Find a nice thermal pad from your favorite electronics store, cut to fit and you should be back in business.
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