I hate to sound like a broken record, but "traces" can mean amazingly small amounts these days due to testing sensitivity. Remember this when you read "traces of [insert toxic metal] were found in drinking water!!".

I don't know what the testing limits for cocaine are, but parts per billion wouldn't surprise me. For some metals, it can be parts per trillion. Ppt would he equivalent to one microgram in a cubic meter (one metric ton) of water.