The best way I've found to locate a leak is to use flourescent dye. I bought a kit with the fluorescent light, some uv enhancing/eye protection glasses, and dye removal spray for about $100 from OnTool. Do a search of OnTool to find their site. You can find the r134 UV dye at any decent auto parts store that caters to the garage trade, not at the chains like autozone.
I had fretted over my 240D until I used the UV dye and homed in on the leak immediately after putting in the dye.
I put dye in my Vette and found the leak immediately, it turned out to be a $10 pressure cycling switch.
To take this approach, you need to be equipped with regular a/c service equipment such as a set of guages at a minimum, and preferrably a vacuum pump to do all your own a/c service.
Best of luck with it,