What I mean is this. Police suspects someone living at a house is running an illegal operation from there. So, they mount a base on a house across the street and from there they observe the suspect, take pictures, etc. The operation can be simple as two cops on a car, from a distance, observing the suspect, 24h.
How do you say:
the cops are doing a _________________ operation
I am talking about the act of doing it hidden.
For example, you have sting operations, where you deploy a bait car to catch thieves, or an undercover operation where you infiltrate a gang, for example. What about this kind of hidden observing a suspect?
I am sure I have heard that a million times on movies, but I cannot remember.