SES is on ! mechanic cant find whats the issue, plz help

[anas]

whats up guys
my SES light came on , took it to my local BMW mechanic & got a p0442 (evaporative emission control system leak) , he looked @ my gas cap & said i need a new one & that should take care of the problem, he didn't have one so i went to the dealer and got a new one , took it to him and he installed it & erased the code , well that didnt work the SES light came back on a couple of days later so i went back to him & it read the same p0442 code, left the x5 with him all day to check it out but he called saying he cant find anything wrong & the only thing he can do to try to figure out the issue is to do a smoke test so i took it back to him a week later & he did the smoke test but still no luck ,he said there is no leaks anywhere !! he said sorry there is nothing else i can do & told me to take it to the dealer .
took it to the dealer & it was the same thing they cant find anything wrong & the only thing they can do to try to figure it out is a smoke test !!!
then a week after that i stoped by my friends used car lot & he scaned it for me & it read 4 codes !!????
p2271 p0140 p0442 p1168
please guys your help would be appreciated