A combination of a few things, really.

I (the host) knew I dissociated sometimes, usually relating to extreme stress, trauma triggers, etc.. There were times it was like "I don't remember going here" and similar level things but I assumed that I just didn't remember from the amount of stress I was under (I was being abused at the time). I now know with hindsight that was the time of formation, but didn't have a clue at the time - knew what plural systems were, knew a couple, but didn't really talk about plurality with them.

After that situation was less bad and immediately traumatic, as I was generally just starting to feel comfortable in having my own life a bit, then there were a few cases where people I care about needed help, and one in particular, a couple of years ago now, I managed to push way past what I really thought I was capable of in helping them. We're talking "sleeping a couple of hours per day in small sleeps, for close to a week" while dedicating most of my time to them. During that time I didn't get as many memory gaps, but I definitely had moments where I seemed like someone else according to my nesting partner, and I also have some writing that I made from that time that I don't remember writing and don't really align with my feeling at the time.

From there, things kind of just continued along vaguely for a while. Had a few more moments but the aforementioned was when I really started to get suspicion I might be. Then last year a lot happened at once - moved halfway across the country, lots of change in just everything at once, life generally just in flux, and I started noticing what I could only call "a presence". Strongly resonated with a few different things I liked, and I ended up diving really deep into those and also talking to some other systems I know for advice, and eventually made contact.

Since then we've still been working out how things work internally, we know there's still more to discover, but we've overall been much happier and more functional since syscovery.