Check the dumb stuff first. Make sure the crankcase vent hose isn't blocked or kinked. If the bike was "in for service" it's possible that something was put back together wrongly, and it's easy for something like this to happen.
If the bike has a history of abuse or neglect, who knows what could have happened. It could have been run low on oil, or wheelied for blocks on end, which will have the same effect.
If there is nothing simple that seems wrong then it's got to come apart.
And supposedly it was okay before the bike was taken in for service? Another argument for do-it-yourself. Keep in mind one other thing ... if the bike was on its side for any length of time, oil could have seeped past the piston rings and gotten into the exhaust system, and it can take quite a while for that to sort itself out.