Blood?
You know ... the stuff which comes out of bodies and sometimes splashes if shot are fired at close range. I believe Mr Bamber was also bludgeoned as well as shot so someone had to get up close and very personal to do that to him.
According to the evidence presented in court this was cold, calm merciless killing which had been plotted for quite some time.
The person who carried it out was well acquainted with the routine of the household ... was familiar with access and egress points down to the faulty kitchen window latch which allowed exit from an apparently locked down house.
The timing would probably never be bettered with all the heirs apparent under the same roof ... including the six year old twin boys.
I lean towards the theory that the best way to avoid being spotted after murdering the family would be using Mrs Bamber's bicycle ... it was all coolly, calmly and premeditatively worked out ... so why not have all contingencies fed into the equation such as allowing for bleeding as a result of brutally massacring five people?