|It's common to get a rash seemingly from nowhere that is caused by touching something (garden tools, clothes) that still has the oil on it. The oil can persist on objects if it's not washed or rubbed off for months or years. This is the most obvious theory for mysterious re-occurrences without any new contact with the plant.
However ... I really think there is something going on here that is not well understood in medicine. I don't think poison ivy is something that receives a lot of research attention, because the most common reaction pattern is well understood - touch a plant, get a rash, be miserable for a couple of weeks, get over it. After all, it's "just" contact dermatitis.
But here and elsewhere there are anecdotes of people getting repeated rounds of reaction, weeks and months later, in the same patches of skin previously affected, which is hard to explain based on re-exposure. I'm personally in week five of repeated outbreaks; in my case it's not just the original area, and it *could* be re-exposure (though I've rubbed down pretty much my whole apartment with rubbing alcohol). Or conceivably steroid rebound, though I only used topical steroids and stopped using them over a week ago.
Nobody has studied it, as far as I can tell, but it seems to me there's something here. Maybe urushiol sensitizes some people's skin to some other substance? Maybe the urushiol metabolites can persist in some kind of dormant state? Maybe there's some sort of biological traces left where the reaction was, and later allergen exposures, even elsewhere in the body, cause heightened response in the originally affected areas? Like some kind of histamine PTSD? I'm not a doctor, obviously. But I keep reading these anecdotes, and I keep getting new rounds of itchy bumps, and I really wish there were more information.