What kind of litter are you using. If its pellets he could just prefer to dump on something soft. If the rug was near the litter tray, then he was in the general vacinity, just favoured a nice soft place. Morrisons do a very cheap recycled paper litter, environmentally friendly but also very soft on the pawsies. You could try that.

There is also a litter called Cat Attract, thats supposed to deal with inappropriate toileting, by having a special mixture in it that makes them want to use the tray. Plus its very small clay peices that clump so easy to get rid of the wees. You could try that, you can get it online.