INFPs are often well rooted in the past, with a good memory of things that have happened historically, and are attracted to "collecting" facts. They sometimes have a strong grasp of family and tradition and may have a great deal of familiarity with their family tree or display an interest in genealogy. They may act as the "family historian" at times, knowing who begat whom and how a particular branch is related. They have long memories. I know one INFP who could recite from memory what year of childhood he first spoke or began walking, including details about what hairstyles he wore at different stages of life! Others like to visit the same places frequently, and let their minds drift back in time to recall previous visits. Nostalgia is often engaged, and they enjoy telling stories from their past. (Introverted Sensing is a cultural norm in the U.S., so it seems fairly "typical" to look to the past this way.) One INFP tells how he enjoys his routines, his habits, doing things by rote. He does the everyday chores of maintaining his home, and these keep him in his comfort zone. Similarly, sometimes INFPs will stay at the same job for years because they're used to it -- even when the job stinks. (A friend of mine stayed at a dreadful job for 9 years!) INFPs sometimes display "body wisdom," and their bodies can be barometers for how they are feeling, whether through illness, food allergies, or headaches. They are often given to taking medications to control their physical being in some fashion. (On a Yahoo Group one time when there was a discussion of medications, it was a veritable drugstore! It seemed every one of them was popping pills to control something.) These are all common ways that Si will manifest in the INFP type code.
I suspect that many INFPs believe they are "J's" because they enjoy the predictability of a routine, and imagine that is "J."
(If you're an INFP reading this, perhaps you can share with me some ways in which you manifest your introverted Sensing...)