I think what is important is the fact that they would not know if it had forensic value without examining it. And this would go for anything else at all they might find.
As far as the water bottle might go, I would imaging they would want to examine contents and create a flow chart from there. What if it contained poison? Or a high concentration of medication? If yes, who put it in the bottle? Who injested it? And so on.
Or, as someone suggested already, if this was GPs , does it indicate GP was left with no water, if a later claim was made about her status when BL left her. I think it needs to be remembered, that the skull remains had not been identified as BL when the area was canvased.
Just that alone makes me think they would have bagged and tagged everything. This could have been a completely different investigation.