Up to no good
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
The phrase is informal and best used in casual conversations rather than formal situations. 
When I saw them giggling with spray paint, I knew they were up to no good.
Pro Tip 2/3Negative Actions
Implies the person is doing something wrong, but the specific action is often not mentioned. 
The neighbors' quiet whispers and frequent glances told me they were up to no good.
Pro Tip 3/3Subtle Suspicion
It suggests a suspicion of mischief or wrongdoing, without proof or details. 
He's been too quiet today. Do you think he's up to no good?