While there can be numerous causes, one that is often overlooked is right over your head - ceiling tiles. Butyric acid is a common by-product of the ceiling tile manufacturing process and can produce an odor that has been described as "sickly sweet", "putrid", and "rancid". The odor can be dormant until the tiles become exposed to certain environmental conditions. Because of the manufacturing process, the strength of the odor can vary significantly from batch to batch, if present at all.
So how do you determine if the ceiling tiles are really the culprit? Well, you could spend a hefty chunk of money on analytical tests or you could go to the break room and grab some unscented resealable plastic bag (1-gallon works well). Put pieces of the tiles in the bag, seal it up, and let it set for a day or two. Take them outside and carefully open the bag and take a whiff - chances are if the ceiling tiles are the culprit you will know it without having spent hard earned money on laboratory testing.
As always please let me know if you have questions. Also, bear in mind that any information provided is general in nature and is not a substitute for a thorough visual assessment and is not meant to apply to specific concerns.