Brake warning light came on and stays on

Just bought a used Buick lasabre. Brake warning light now on, I brought to dealer (under 30 day warranty) who checked it over for leaks, found none - checked a level in master cylinder? I have firm pedal, car stops and all that but I don't want to drive it with light on. Mechanic at Yorkville Auto says it should be fine, he could not find anything wrong with it, he did not do overly throro check to me - checkled line for leaks and found none as stated - but is now insisting it is connected with ABS (that light also on but I was not concerned with that one)
he said come pick up th car as he can't find anything wrong with it. So - I am but am also gonna take it to TP or a brake specialist and have em troubleshoot then return to dealer for repairs. Before I do that - any suggestions as to why if they looked can't find teh issue? What could it be if they checked all that - are they skipping or missing something? I realize when you sell a car cheap you don't want to spend money repairing - but it is also a safety issue as far as I am concerned.