It is understood the pensioners told security staff who eventually apprehended them that they were only following the satellite navigation instructions to take them to Loch Lomond. A witness said they had the system set to 'foot passenger' mode.
The airport, which is playing down concerns over security, denied reports the couple's car had managed to get within yards of the runway and no investigation is being carried out into what happened.
However, one insider said the incident raised concerns about the breach of security so close to the 2014 Commonwealth Games.
It comes six years after the Glasgow Airport car bomb attack led to a series of security shake-ups and a reshaping of the layout.
Airport officials initially said the German tourists' car managed to pass through the North Gate security checkpoint entrance while following an authorised vehicle but was then stopped "immediately".
Police were alerted as a "standard practice".
Later, the airport said the car had managed to travel 70ft beyond the perimeter fence, by which time gate guards had contacted airport staff about the breach.
The tourists would have had to travel another 300ft to get to a taxiway, the path used by an aircraft to get to the runway.
It is understood they had just landed at the airport on Monday morning and collected their car from the rental firm Enterprise when they went on their unplanned detour.
A witness said: "They said they were looking for the road to Loch Lomond, and the satnav was taking them through the airport, but they should not have got through security. They had their satnav switched on, but they had it switched on for foot passengers.
"The place was in uproar about it, thinking the whole thing was funny, but they really should not have got through security like that. With the Commonwealth Games coming up it is very concerning."
Airport security is regularly audited by the Department for Transport and the source added: "As far as we are concerned this has shown people were being vigilant and things are working well.
"There was no risk to aircraft or the security of the airfield because the vehicle was monitored and followed at all times. The security guard alerted the control centre as soon as the vehicle entered the perimeter.
"All over the entire footprint of the airport, whether in the terminal or at the gates, people are told if anyone comes in behind you, you notice anything suspicious, you are told to challenge them."
A Glasgow Airport spokesman said he was unaware there was any review or investigation of any sort being undertaken into how the car got through the perimeter fence.
The airport said in statement: "At approximately 10am a vehicle drove past a security checkpoint. The vehicle was approached and apprehended by airport staff immediately, and escorted from the area.
"The incident has been investigated and we are satisfied the proper procedures were followed. At no time was there any risk to aircraft or the security of the airport."
Glasgow Airport was targeted by terrorists in June 2007, when a blazing Jeep Cherokee loaded with propane canisters was driven into its doors.