The only semi-erect tent I'm familiar with is the one formed by my duvet when dreaming about Ana Moura.
As I suggested before, maybe June dropped the bible open at pages 656/657 onto the bedclothes when she was shot in the neck. After feeling the wound and as she was trying to get out of bed, her bloodied right-hand palm came into contact with the top of page 657, together with a couple of small spatters lower down on page 656 and the book's fore-edge. The bible then closed on itself, perhaps through agitation of the duvet/bed clothes (or even later by Bamber himself as he got hold of it after the blood had partially dried), which caused the faint mirror images to appear. When Sheila was dead, he re-opened the bible at the stained pages and placed it on the bloody carpet near her arm, hoping the police would think this was what caused them... until Caroline came along 30 years later to show otherwise.