Could be any number of things ...
It's just early days and as mentioned by others it will take a short while to settle ... Some spend DAYS just crawling around the Viv looking to find an opening .
I'd leave it alone for at least 7 days before even thinking about offering a "warm" defrosted mouse . Check around this forum for the best ways to offer it . Some suggest not handling until it's fed twice .
As to staying outside the hide - maybe he just doesn't like it as warm as 90F ...they are all slightly different..maybe it's much higher than 90F ?!
What are you using to take the temps under the hide ?