It depends what the drug test was looking for and what criteria were applied to the results. The only drug in Red Bull is caffeine. Caffeine's obviously not illegal but it can affect performance in various ways. Some drug tests might not look for caffeine, simply because the people who've ordered the test aren't bothered whether it's there or not. (We have random drug tests at work but, to the best of my knowledge, caffeine isn't one of the things which the tests look for. So, as long as an employee hasn't been using illegal drugs, he'll never have a positive test however much caffeine he's ingested). Other tests do look for caffeine. (Athletes have to be very careful about drinking coffee, Red Bull or even tea. An excessive amount of caffeine in these tests will give a 'positive' result).
If a test has shown a positive result for something other than caffeine (e.g. cannabis) then it will have no connection to the amount of Red Bull, etc., that has been consumed. (Cannabis will normally show up at least a fortnight after it has been used. It can sometimes show up well over a month after last usage).
Just a last thought. Most health advisers give a guideline safe figure for the maximum number of drinks containing high levels of caffeine (e.g. coffee, Red Bull, etc) as a total of 3 drinks per day.
Chris