The silicone cases do give these cheaper phones a more solid feel (they certainly improved the feel of my San Francisco) but the blob was back again yesterday despite the case and screen protector I have been using for the past few days. Once again I was able to get rid of it after a while by removing the back cover, battery and SIM card and pressing about a bit from that side; particularly in the area of the SIM card slot. The blob changed size and shape as I did so and, after a bit of persistence, disappeared completely. When it disappears, it looks as though it is being 'sucked' into a tiny hole, which seals up without trace. In other words, the blob shrinks rapidly and disappears, as opposed to the whole shape vanishing instantly.
I'll try to capture it on video if it happens when I am at home. Typically, though, the only time the phone is in my pocket is when I am out and about, so have no access to cameras.