Is the target of Sanctuary aware of the spell effects?

I am playing a Cleric and regularly use the spell Sanctuary on low life teammates while telling them to back away to safety (short call so I can heal them up and they will stop attacking and avoid breaking the effect).

I had one time when I had to choose whether to say to my Sanctuary target to run away or alert another character from a danger that only my character perceived. I choosed to alert my target but it lead our Wizard to be critically stabbed (he’s fine, merely a flesh wound)

So I was wondering, is the target of Sanctuary aware of the spell effect or should I continue telling them not to break the effect ? If they are aware of the effect, do they just "feel protected" ? Do they also get the intuition that being harmful to another creature will break this protection ?