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Originally and chiefly British English, the colloquial expression spy in the cab designates a tachograph—i.e., a device in a motor vehicle for automatically recording its speed, travel time and other information.

This expression occurs, for example, in the following letter to the Editor, by one Harold MacKenley, published in the Durham Advertiser (Darlington, County Durham, England) of Thursday 28^th October 2021 [page 20, column 2]:

Lorry drivers have been undervalued for decades. Dismissed as not being clever enough to do anything else so they drive. This is, of course, erroneous.
Truckers, along with their cousins, coach and bus drivers, are the true professionals.
These lads and lasses have to contend with the police, the traffic commissioners, roadworks, detours, the weather, forklift drivers too slow in loading or unloading, the gaffer on the phone every ten minutes as to why this load has not been picked up or that load delivered and the spy in the cab (tachograph).

The earliest occurrences of spy in the cab that I have found indicate that this expression was coined in Manchester (a city in north-western England), in September 1968, by lorry drivers who opposed the proposed introduction of the tachograph into lorries.

This was first mentioned in the following from the Manchester Evening News and Chronicle (Manchester, Lancashire, England) of Thursday 19^th September 1968 [page 17, columns 5 & 6]—here, the expression that is used is not spy in the cab but robot spy:

PROTEST TO WILSON ¹ OVER ‘ROBOT SPY’
BY OUR LONDON STAFF

Salford lorry driver Mr George Landing went to 10 Downing Street today, to hand in a protest letter from drivers about a proposed “robot spy” for truck cabs.
Mr Landing is secretary of the North-Western Transport Drivers’ Action Group, which has already called for a protest strike by drivers in the area.
The drivers are angry about part of the new Transport Bill, which calls for the introduction of the “tachograph” into long-distance lorries.
The tachograph is a “black box” meter, which records when drivers stop and start en route.
In their letter to Mr Wilson, Action Group members say that at present drivers are controlled by rules of law and common sense.
They feared that if the tachograph came into use it would tempt some drivers to carry on even when they were over-tired.
The drivers also feel the meter is an infringement of their personal liberty, akin to “Gestapo tactics.”

¹ The British politician Harold Wilson (1916-1995), Baron Wilson of Rievaulx, was Leader of the Labour Party from 1963 to 1976, and Prime Minister of the United Kingdom from October 1964 to June 1970, and again from March 1974 to April 1976.

These are, in chronological order, the earliest occurrences of the expression spy in the cab that I have found:

1-: From the Manchester Evening News and Chronicle (Manchester, Lancashire, England) of Monday 23^rd September 1968 [page 9, column 5]:

Ignore ‘spy’ strike, drivers told
BY BRIAN HOPE
Our Industrial Correspondent

Union leaders today urged more than 25,000 North West lorry men to ignore an unofficial call for strike action over a “robot spy.”
The strike, due to begin at midnight on Saturday, has been called by an unofficial action group in protest at a proposal to install tachographs in all commercial vehicles.
These machines—known to lorry men as “the spy in the cab” or “the hidden policeman”—record the speed of vehicles, the time taken on journeys and the length of intervals when the vehicle is stationary.
They will become compulsory if the Transport Bill is approved without amendment.

2-: From the Manchester Evening News and Chronicle (Manchester, Lancashire, England) of Monday 30^th September 1968 [page 7, columns 7 & 8]:

Drivers walk out in ‘spy in cab’ row
By BRIAN HOPE
Our Industrial Correspondent

Drivers at three of Manchester’s biggest road-tanker depots were on unofficial strike today over a threat to put a “spy” in their cabs.
[…]
The Action Committee chairman Mr Bill Axon told the men: “We have started the ball rolling.”
[…]
Commenting on the “spy in the cab,” Mr Axon said: “As far as your future driving life is concerned you are under sentence of death.”

3-: From The Guardian (Manchester, Lancashire, England) of Tuesday 1^st October 1968 [page 5, columns 2 & 3]:

More tanker drivers join strike against ‘spy in the cab’
By our own Reporter

The strike of road tanker drivers in Manchester spread to Leeds ² yesterday and about 1,000 men have now stopped work.
They are protesting against the proposal in the Transport Bill to install a tachograph in lorries—a device which records speeds, length of time taken on journeys, and periods when the vehicle is stationary. They regard the device as “a spy in the cab” and say it will not help road safety because drivers may be tempted to drive faster to complete journeys on time.

² Leeds is a city in Yorkshire, a county of north-eastern England, on the North Sea.