It means, "You probably don't know this but..." (used a lot in TV advertisements).
You wouldn't say it to your supervisor, in fact I don't think I have properly heard it said this way outside of TV adverts and telling a friend something, in which case 実は... (or one of the examples at the bottom of this answer), are normally used.
Also, it isn't normally used outside of most advertisements because in some contexts it can portray a sense of the listener being out of the loop and thus not included from the goings on in the topic (which in terms of an impersonal advertisement is exactly what the situation is assumed to be).
It may also be seen as attempting to insult the listeners intelligence usually by stating obvious facts (hence why you shouldn't use it with your supervisor).
You probably don't know this, but Japan is an island nation.
Saying that, it does still appear in everyday conversation.
Here are some examples of its use.
[TV Shopping Channel] (Note: Not an actual advertisement. Actual company names have been changed)
Your normal Vacuum cleaner just can't get all of the dust and things in your carpet can it? You probably didn't know this but Daisan's new products have Cycle Technology built in! Cycle technology can reach even deeper than a conventional Vacuum! ...
[Talking with friends]
A: I haven't met Tomo in a while. I wonder what he is up to?
B: Ee? You probably didn't know but he married a foreigner and moved to America.
＊ It is to note that when talking with friends, it is more common instead of あなた（など）は知らないと思うけど to use something more colloquial and with a somewhat surprised nuance.
i.e. ええ？知らないの？ OR あなた知らないよね？ OR 聞いてないの？ etc.