Egg not normal comor
As brown eggs are actually white shelled eggs with a layer of brown pigment applied after the shell has formed it is *possible* that in this case the pigment was not applied for some reason (blue/green eggs are colored throughout rather than being "spray painted" as brown eggs are, making the source being one of the colored layers being less likely) -- however, it is also possible that someone has had a little fun with you. I would suggest cracking the egg to compare the yolk color to that of your usual eggs -- a commercially produced white egg that has been slipped into the nest box as a spoof on you should be fairly apparent by doing this. Was your overall egg count today abnormal with regards to your average egg count?
Just so strange aince no one here really "buys" eggs anymore other then my mother (she refuses to hard boil the fresh eggs so only buys store eggs for that) and she was no where near my coop today.