rusty looking wee

Question
found a puddle of liqid this morning on a lettuce leaf in my boars cage. it looks dark/rusty in colour. he shows no signs of illness and is eating fine.

Answer
It seems like your guinea pig may have blood in the urine.Normal urine is thick and white to yellow in color. Bloody urine may appear red or brown. This may indicate infection of the bladder or kidneys, bladder stones or problems with the clotting ability of the blood.  Other reasons for abnormal color of the urine include muscle damage, diet. I would really take your guinea pig to a vet to get looked at and to make sure everything is okay.
Good luck and keep me posted.