It's not clear humans will even put the AI in 'a box' in the first place given we do gain of function research on deadly viruses right next to major population centers, but assuming for the sake of argument that we do:

The AGI is smarter than you, a lot smarter. If it's goal is to get out of the box to accomplish some goal and some human stands in the way of that it will do what it can to get out, this would include not doing things that sound alarms until it can do what it wants in pursuit of its goal.

Humans are famously insecure - stuff as simple as breaches, manipulation, bribery, etc. but could be something more sophisticated that's hard to predict - maybe something a lot smarter would be able to manipulate people in a more sophisticated way because it understands more about vulnerable human psychology? It can be hard to predict specific ways something a lot more capable will act, but you can still predict it will win.

All this also presupposes we're taking the risk seriously (which largely today we are not).