After thinking I might not see my little alien blob again, I found it! It's settled on part of an old, eroded shell in the corner of the tank. Hopefully it'll do well; it picked a spot that should be pretty well protected from things, including my other BTA clones. While my clones never sting each other, bigger anemones aren't above eating smaller ones when they're adrift in the current.
So I had a bit of a cyano bloom in the tank and realized that, ah beans, I should've put some phosphate remover in the canister when I had it open last as a precaution against that (algal and bacterial blooms are common following dismantling and reassembling a tank like I did). Well, time to crack open the canister again and add some of that. Surely can't be any more surprises right? Uh...
Yes, that's a third tiny BTA, and the second to be found in the canister filter. And yes, the other two are accounted for in the tank - this is definitely a new one. At this point I am really starting to doubt my original hypothesis that these are the result of a messy split and anemone bits getting sucked into the filter. I never saw a split messy enough to plausibly produce multiple fully formed little anemones out of stray bits, and I look at this tank every single day so I'm confident I would have seen something like that if it happened. I did have a mysterious, lightly cloudy tank event a couple months ago. I assumed at the time it was my macroalgae doing its thing but I realize now that I never did have to pull any dieoff out after that. Perhaps it was a spawning event, but I am hesitant to re-hypothesize that these tiny nems are the result of a spawn, in part because all my clones are from ONE original anemone - which as far as I know will have been either male or female. I have since read that clones of some anemones can switch genders, but haven't seen BTAs specifically discussed for that. Some other anemones like rock flowers are well documented as spawning in tanks with new anemones surviving the larval form and settling...but I've also never heard that of BTAs in an aquarium. Spawning BTAs in a tank does happen but I haven't heard of successful larval development from them. Aiptasia anemones have another reproductive method and will send small buds off of the base of their foot to form new anemones, but I'm not aware of that being a reproductive method in other species like BTAs that split down the middle to divide.
I did briefly wonder about spawning as a possibility when I saw that first one. Given I've never heard of it happening and can't find anyone else on the web reporting it, I just thought no way - it would not be in the tank that I've just let go feral and grow into a tangled mess for a month leading up to being moved, it wouldn't happen in a nano let alone a weird minimal equipment one like I've got, etc. But the alternative I came up with just doesn't work out with three of these things and one being right in the tank where I would have seen a dangly bit of anemone skin waving around if that was it. That leaves spawning and budding, and there seems to be zero evidence of Aiptasia-style budding in BTAs. To invoke my favorite detective, when you have eliminated the impossible, whatever remains, however improbable, must be the truth.