There's a mod that does that in the workshop called "True Concealment". Personally I don't think it makes too much sense in most of the missions, as the briefing states that the activity with the timer is occuring before XCOM deploys, but it's still a decent mod. There's also a mod to increase the timers as a whole.
If anything, the fact that they decided to focus on modability is my favorite part of XCOM 2. They give you access to so many scripts and functions that, realistically, most modders will never need to touch a substantial majority of them.
EDIT: There's 3,465 script files weighing it at 25 MB!
