If they are powered down and connected to the larger ship, the sensors wouldn’t pick up either a transponder, an independent contact, or a power signature. Once they’re past/under the sensor sweep, they can drop off and move undetected. And “undetected” can mean obscured by so many other signals that they may as well be invisible.
As for “free parking,” if it’s unattended and they just set down like they know what they’re doing, they might be able to away with it. I don’t remember the context of their landing, however, and things may have happened off-screen.
Police was definitely a poor choice of terms. We’ve seen police droids before, and they are always marked. These were not, and were armed with E-5s, which as far as I am aware do not even have a stun setting. “Security” would have probably made more sense, I don’t know why they were called “police.”