Sometimes I have seen native English speakers’ uneasiness when they heard “I don’t know / I have no idea.” They seem to use the phrases carefully, avoiding to say it as much as they can. If they have to say “no idea,” they will try to be more polite and followed some words like my friend, mate, buddy.
Is there some hidden meaning or implication with these phrases? Perhaps a colloquial meaning that implies rudeness or unkindness?
The words themselves don’t seem inherently rude or unkind, yet native speakers seem to treat them as such. Am I missing something?
There isn’t any particular special situation in the English usage of such phrases (that express lack of knowledge), except a general reluctance that some people have admitting they don’t know know something – in English that is more dependent on the person than related to language or culture (though there will be some English speaking regions where it is more common of course).
Regarding the two particular phrases you mention, I would tend to avoid “I have no idea” unless talking with friends – it can sometimes come across as flippant or dismissive.
The second part of your question about politeness is true – particularly with strangers, many native English speakers will add in an apology.
For example, when asked for directions, I will often reply “Sorry, I don’t know”.
But again, this is rather personal – there is no rule, and it is possible to be perfectly polite with all the phrases, by adjusting tone and body language.