Well, if you're going to adopt an ankle-carry method for plainclothes or off-duty, at least make sure it's concealed ...
The other day a friend and I met at a Starbuck's coffee shop for a few minutes, and sitting inside the coffee shop, in shirt & tie (lacking jacket), was a fed agent known to my friend. Exposed for all the world to see, under his hiked-up left pants cuff, on the "outside" of his left ankle ... was his holstered weapon. We don't often see that many uniformed cops stop in that particular store, let alone fed plainclothes, so it's not like it would be a common sight to see an "unintentionally exposed" weapon, either.
Ankle carry does work very well in the respect that it places a weapon within easy reach while "trapped" behind the seat of a marked or unmarked car.
I don't carry a secondary weapon at the moment ... (although I've carried a couple of primary weapons when I felt the situation called for it) ... but back when I carried one I had it in my strong side jacket pocket. If I were to carry one in my present assignment it would be a 642 in a pants pocket.
I've seen secondary weapons carried in many places the owners couldn't easily reach even under normal conditions, and it makes you wonder how they'll fair when they're under stress, and/or fighting for their lives in some awkward maelstrom of limbs and unanticipated obstacles banging into them during a fight, doesn't it? I suspect that most of the uniformed people I've seen carry secondary weapons select their method of carry more for day-to-day convenience, than for the practicality of being able to actually reach and present the weapon ... and under even worse circumstances than they experience while presenting their holstered service weapon.
I'm not saying that ease of carry isn't a consideration ... especially off duty ... but familiarity & practice are perhaps even more important with carry methods employed for secondary/off-duty weapons.