Low light focussing is a camera problem. I am now reading the manual of the 1DIII and makes that clear:
" Autofocus can fail to achieve focus(the focus confirmation light blinks) with certain subjects such as the following:
SUBJECTS DIFFUCULT TO FOCUS
--Low-contrast subjects, e.g.. blue sky, solid-color walls, etc.
--Subjects in low light
--Extremely backlit and reflective subjects, e.g., car with a reflective body, etc.
--Overlapping near and far objects, e.g., animal in a cage, etc.
--Repetitive pattern, e.g., skyscraper windows, computer keyboard, etc.
In such cases, do one of the following:
(1) With one-shot AF, focus an objectat the same distance as the subject and lock the focus before recomposing.
(2) Set the lens focus mode switch to MF and focus manually.?
Canon EOS 1D Mark III manual, page 86.
Just because it's sometimes a camera problem, doesn't mean it's not also a problem specific/inherent to this lens.
I was shooting in low light long before I bought this Sigma.