Got an Answer?
Your question is a waste of common sense.
You are a suspect. Not convicted. They aren't tarnishing your future based on suspicions. They need to build a case.
Rule 1. Be an honest person
Rule 2. Live by the 10 Commandments. They're a pretty good guide thru life in western society.
Under cover surveillance. You might run if you knew they were on to you.
They need to build a case
So they don't destroy evidence, so they don't flee, the police might be following them, hoping to be led to evidence, etc...
One would have to have an idea given they would usually be questioned plus the obvious that they would hide evidence, create alibis and run!