As someone else pointed out before, this was probably nothing noteworthy. Artists always make multiple layers and improve upon a base, this is just standard technique. This isn't some Da Vinci Code hidden Illuminati story.
They also reuse canvases because they tend to be poor and hey, they've already got this perfectly serviceable canvas sitting around with something they don't care about.
Couldn't they be pentimenti?
pics or it didn't happen (i mean the real layer back light see-through stuff).
preferably before and after substraction of what can be seen without the backlighting..