A couple posters here have hit the answer to your question.
The answer is YES, lenses may have different abilities to resolve detail (defined as lines on the sensor at a given contrast ratio), depending on the focus distance.
As of yet, I know of no site that actually measures this characteristic of Canon or any other lenses.
I've certainly noticed this with a few lenses, typically zooms, where they will do a better job at close distances than at or near infinity. I had a 17-40/4L that comes to mind first.
With the minute inconsistencies of manufacturing tolerances, various copies of a given lens model may exhibit more or less of this tendency as well with further variation possible at different focal lengths.
It sometimes seems that more low cost zoom lenses perform this way than others, likely because they're compromised to work better at the closer "people-are-the-subject" distances they're used with when kitted with consumer bodies.
Pro level lenses aren't exempt from this effect either, many workhorse zooms are a bit less sharp near minimum focus distance.