How is leaving a telephone on the ground a "classic anti-surveillance technique"?
What surveillance does leaving a telephone on the ground while walking away counter?
If the mobile phone is compromised and acting as a covert listening device, also known as a "bug". To walk away is so that conversations are too far to be properly heard.
Nobody needs to actually use the phone and make a call for such surveillance to happen, it is just a thing that sits there and collects every sound around it.
Examples
Remotely turning on cell phone microphones, from 2012:
a judge published a statement that the FBI's established practice of remotely turning on cell phone microphones to monitor conversations was indeed legal.
Source: What are prudent security precautions to take for business travel by people with access to sensitive info?
Even smartphone gyroscopes could be re-purposed into microphones:
When the researchers tested their gyroscope snooping trick's ability to pick up the numbers one through ten and the syllable "oh"—a simulation of what might be necessary to steal a credit card number, for instance—it could identify as many as 65 percent of digits spoken in the same room as the device by a single speaker.
Source: https://www.wired.com/2014/08/gyroscope-listening-hack/
Further examples: https://www.schneier.com/blog/archives/2006/12/remotely_eavesd_1.html
Other parts of your question
Without further clarifying details from the lawyer, most likely:
I am curious as to how leaving a telephone on the ground in the street
The ground and street are just due to where this occurred, not specific to understanding the walk-away approach.
is considered [a] "classic anti-surveillance technique".
The classic is an adjective to describe the anti-surveillance technique, that it "has been done since long ago", implying "everybody" "should already know this". Then the lawyer can say that it is "obvious". From there, how such "obvious" things were missed, so that along with satisfying other requirements, the lawyer can establish that there was negligence and win the case.
Leaving it on the ground is irrelevant. Leaving it is the important bit.
As mobile phones can be subverted to act as microphones, leaving them behind when discussing sensitive topics is a way to be sure they have not recorded or transmitted your conversation. Actions like this, or taking in very loud environments etc, have been used by spies and criminals for years.