LoriP. Random smells can occur without there being a serious medical problem. As far as I know, I’ve never heard of burnt toast smells occurring before strokes. This seems like an internet/urban legend to me.
However, simple partial seizures (as opposed to generalized tonic-clonic seizures where people collapse unconscious and contract their muscles on the floor) can be associated with an aura of unusual odors, as has been mentioned above. Some of these simple partial seizures can transition into more complex partial seizures, and you may not even know it if they occur when no one else is around to witness the event due to amnesia. The fact that you remember these smells suggests that IF it is anything, it could be a simple partial seizure. That said, I would quiz anyone around you after you notice these smells to see if you are behaving funny, like staring off into the distance (i.e. daydreaming). If so, that suggests a complex partial seizure.
There are other things that could cause this, ranging from benign to more serious causes. I concur with the other flutherers who suggest you talk to your doctor about this.