Me too. The being mistaken for a local (in across sea English speaking country) in terms of speaking was both an honor and a surprise as it's not as if I was trying. Like you, I pick accents up quickly -- had been travelling a week. The only "american" accent I'd heard was my son's (I don't travel to insulate myself in a cocoon of familiarity. What would be the point of that?!?!?! So, local buses etc.) Aside from speaking, I was approached SO many times by people seeking information. It was weird.
I was DEFINITELY hypo on that trip, though I don't know that it had any bearing on the experience.
You could be onto something there. I didn't start with the local accent of where I grew up (Not heard at home) and it might have helped in replicating a mixture. From birth, that is how language is learned, no? We parrot what we hear. If we can retain the ability to really keep hearing in that open way, it stands to reason we could assimilate sound (and language -- I've done 6, though 2 of them only for a semester.) The more places, the more sounds, till it's basically an indistinguishable blend. Yup, I've moved a lot.

Talking-wise, I am always bouncing into characters and accents just for amusement.