Yeah that's what I ended up thinking because who wants to damage their sub just by cleaning it? I only happened to see it when some stray sunlight filtered in and cast a very unforgiving light on the sub.

A: Epic 80 * 600 / Integra DTC-9.8 / A1400-8
V: Samsung HL-S6187W / Tosh HD-XA2 / PS3 / Oppo 970HD