I would recommend you lock your domain name down. there should be a facility for that from where you have it coming from, sounds like they are hijacking your domains ip address thats why you couldnt find it believe it or not it is quite common I host with yahoo at the moment and they recognised that problem early but some hosts charge for the privilage of locking your domain.how the hijackers do it is by going into any whois database check the domain and bingo your ip for that domain shows up if its not locked they piggy back off your servers and will catch half your traffic in overflow and misdirections, bad links etc.etc. its either that or your just getting an echo from a search engine.check your bandwidth and see what pages are higher than usual traffic usually finds where its coming from.
hope that long winded explanation helps