As an INFP or ENFJ... if somebody called me a sensor?
1) It depends on who said it, the manner by which they said it, the degree of seriousness by which they said it, and so on. Sometimes, I will decide to joke back to test what's going on, or to see if they meant it. Other times, I will joke and tease them if I think they meant it to push my buttons, and try to refrain from going into a heated talk, which I have the strong tendency to do. I can't help it.
2) They could be projecting themselves in me. They could have characteristics of an N personality although they are S, and really value these characteristics in relation to being an S that when they see it in me, they will class me as an S as well.
3) I am very good with multi-tasking and managing the environment to the Tee. I can be active till the end. I like to get things done, most of the time, although I have my share of delaying, and as a result, regrets. Thus, to be called an S, would mean they see me doing what Sensors identify with the most. I have noticed how in tune they are with the "real" world, and are always up for activities. If I ask them to do anything with regards to helping me out in this "real" world they just know how it all happens, because they are used to it.
4) The way people perceived me in the past, may be a reason as to why they would call me an S. They are associating me to my past behaviour or what I'd rather say "their perceptions of me in the past", hence, taking me as a sensor. Often what I've noticed about sensors is that they need convincing, mostly practically, to realize the difference, unless I go all abstract in my conversations which makes them go HUH? and not necessarily YOU'RE AN INTUITIVE?! More like "weirdo, pfft"... to know an intuitive you have to relate to one, I suppose.
5) I could be trying to fit in or keep up with the crowd, and be mistaken as a Sensor, although this is very unlikely as people still see me as me. However, if I really bury my true self in a public setting...

A similar situation is me not opening up my "N" side to people and preferring to joke and go all hilarious and social instead. So if anyone brings up something N, I will tag along with it, but not for so long unless we were conversing one on one.
6) One major reason why I would be called a sensor, or anyone for that matter, is being avoidant of something in order to regain something else. What I mean by this is that one would need to operate like a sensor without losing their true N side, only because it seems like an effective tool to deliver what they really stand for, because it simply works with perhaps the majority of people. At times, this sort of tool is what one will be comfortable with, and begin to appear like a sensor. I may want to deliver a good point that may be of use to people, or a talent/ability of mine that I'd like to see in action and for others to acknowledge as well.
7) Sometimes, I DO prefer Sensor traits. And I will go with how I feel. Deep down however, N runs in me, and I find myself clashing with other sensors despite preferring sensing traits in certain situations.
8) Those rare moments when I want to be anything but me, out of exhaustion or misunderstandings or not just getting up and doing it, I will put myself in a situation where I can be somebody otherwise -- and although I would still come off as an N, sometimes in such cases I will appear like a sensor.
These are some of the possibilities as to why people would call me a sensor, or even think that I am. Do I find it offensive to be called a sensor? Not if it wasn't intended. In fact, I'd get intrigued as to know from the person who said that I'm a sensor, why do I come off as one? However, I find it important to make them understand and/or realize that I'm an intuitive or at least there will be points where my strongly carved values and stances in life could clash into their's and that they don't have to assume I am like them in most instances. I do worry that what if they expect me to be like them or hope that it'd be the case. I am very open to learning and growing with people, be it sensors or intuitives. However, I will not sacrifice my true-self, despite having the tendency to set myself aside from time to time. Nothing's an offense, except if it was intended, and sometimes I mistaken somebody's intention to mean one thing when it meant something else.