I've heard so many stories about snakes that just randomly soak. Everyone tends to freak out and wonder if 'something is wrong'. If she's just soaking casually, I wouldn't worry too much. You say her tub is secure, she has a secure hide and might be shedding. I'd say that those are all likely reasons she could want a soak.

If she's in there for a prolonged period of time, then I'd worry. Otherwise, she's just having a dip for some reason. I wouldn't fret it if you've checked your husbandry and can't find anything immediately wrong.