What does "Can I help you?" really mean?

I am a non-native English speaker. Whenever I travel to the States and I sometimes hear the question, "Can I help you?"

At first, I thought it was a sincere eagerness to help me. However, after a few incidents, such as accidentally entering a wrong meeting room once, I learnt that this question can also mean, "I don't understand who you are or what you are doing here."

So now I am confused if there is also a negative connotation associated with this question and if there is, then how do I understand from the context if this question is asked positively or negatively.

Are there any thumb-rules one can follow to deduce if "Can I help you?" is said in a positive or negative way?

I am sorry that my answer has to be long.

If asked this question, it is helpful to think about on whose territory you are and why.

In an open shop you are on territory where your role is to buy, theirs is to sell; you may safely assume that they want to help you buy.

On a public street you are entitled to be there and if you looked puzzled you may safely assume a genuine offer to help you find your way.

If you are on private ground they may ask you the same question with the same helpful intention (you may merely have lost your way), but there is also the possibility that you have entered the ground with purpose; you must recognise that you are not entitled to be there and should take the question as an invitation to leave, or to establish entitlement (“Yes, I understand from your advert that you want to sell your car”). Otherwise, apologise and leave.

Similarly, if you enter a room where a meeting is taking place, The question implies that you must recognise that the room is territory where you are not expected or entitled. Only with good reason (“yes, I am here to tell you the hotel is on fire”) can you establish entitlement. Otherwise, apologise and leave.