Insidiously (adverb) – awaiting a chance to entrap

https://www.merriam-webster.com/dictionary/insidious

“The incident left her profoundly disquieted. Her old unhappy questioning mood came again upon her, insidiously stealing away more of the contentment from her transformed existence.” (pg.113)

The characteristic of non-optimistic questioning appeared again as if it was waiting for the chance to come out and seize it’s moment. Which wounded up making her unhappy about her life.