It's not very likely, unless you're a heavy sleeper who happens to sleep in unsafe places regularly - or your partner at home is up to no good.

Also, implants are kind of finicky with respect to reader placement, because they're sitting under a layer of skin full of conductive water, and they're usually not symmetrical, so the reader has to be positioned a certain way to score a good read. You as the implant owner know "the move" (in fact, it quickly becomes second nature and you never think about it anymore) but unless you explain it to someone or they know about this shortcoming, they'll have a hard time getting a read. That's assuming you don't wake up because someone is touching you, because the read range is very short - like 1/4" when the reader is ideally placed - and you don't hear the loud bing from the cellphone.

But yeah, you're correct: strictly speaking, if you have a good memory, a long and complicated password - or a mental "recipe" to make one - and a healthy habit of changing passwords regularly is better. Or better: a password in your head and an implant as a second factor.