Wondubious

Something that appears wonderful or impressive on the surface, yet carries an undercurrent of doubt or suspicion. The feeling that something is too good to be true — simultaneously exciting and unsettling.
From wonderful + dubious.

I told him I'd cover his shift, no worries, happy to help — and watched that wondubious look creep across his face, like he wasn't sure whether to thank me or check his back.