The shock revelation was the talk of the Monaco Grand Prix weekend, with Ferrari and champions Red Bull putting in protests that could see Mercedes end up in front of a governing FIA tribunal charged with breaking a ban on in-season testing.
Secrets are hard to keep in the Formula One pit lane and paddock, accessible only by swiping a coveted pass through an electronic turnstile, where there is generally more spying going on than in a James Bond movie.
Teams have in the past paid photographers for close-ups of rival cars, while reporters are forever chasing a steady drip-feed of rumours and deliberate leaks designed to unsettle competitors.
Little escapes the eyes and ears of mechanics and the 'truckies' whose job it is to set up the palatial team 'motorhomes'.
In-season testing has been banned for years, a move agreed by all teams to keep costs down, although Pirelli can conduct their own limited tyre tests with an older car.
Yet Mercedes and Pirelli managed to hold a three day, 1,000 km test at Barcelona's Circuit de Catalunya, in the week after this month's Spanish Grand Prix, with a 2013 car and seemingly without anyone noticing.
Mercedes suggested that was hardly their fault.
"We left everything there. The garages, the buses, the trucks, all the engineering offices. Nothing was secret. We didn't Twitter. Should we have?" said their motorsport head Toto Wolff.
Dismantling a multi-story motorhome, such as the glass-fronted one used by McLaren or the 'energy station' favoured by world champions Red Bull, can take days.
Surely, paddock insiders wondered, there must have been someone around other than Mercedes and Pirelli employees to wonder why one team was not packing up and what Lewis Hamilton and Nico Rosberg were still doing there.
"It's remarkable nobody knew a current car with current drivers was running around on the Barcelona track," agreed Red Bull principal Christian Horner, who found out on Saturday after news leaked out of a drivers' meeting.
"You have to say Mercedes are good at holding a secret. Most other things you hear about, somebody is going there or leaving, but they have managed to keep this one remarkably quiet," added Horner, whose team has its own @redbullf1spy address on Twitter.
Mercedes could now face heavy sanctions. If the Monaco winners are found to have flouted the regulations, they could be stripped of points, fined or even excluded.
Should they be cleared, that would open another can of worms since it would open the door for others to carry out tests and might lead to the renewal of a 'spending war' that the sport can ill afford at a time of rising costs.
Ferrari have pushed repeatedly for in-season testing to be re-introduced and team principal Stefano Domenicali said that was one of the reasons for their protest.
"If this is possible we will be the first to raise our arms to make sure we can do the same, because as you know Ferrari has always been very much pushing to try to do in-season testing on the track," he told reporters.
Apart from resolving the legality or otherwise of what Mercedes have done - and Mercedes were adamant that they had done nothing wrong - considerable damage has already been done.
Horner and other principals expressed concern at the lack of transparency - with neither Pirelli nor Mercedes notifying other teams about the test - and also suggested Mercedes had done it to gain an advantage.
The latter accusation, denied by Pirelli and Mercedes, could prove the most pernicious if the 'Silver Arrows' perform strongly in Canada in two weeks' time when modified tyres are due to be introduced.
Voices in the paddock were already suggesting on Sunday that any Mercedes championship challenge could be considered tainted, even if Rosberg's Monaco victory had been flagged up long before the test given his pole positions in Bahrain and Spain.
Horner said Mercedes's behaviour was certainly 'underhand'.
"Whenever you run these cars you are learning - about reliability, about the mechanical side of the car, about how the tyres act and behave," he said. "For Mercedes to claim they didn't benefit from that test would be difficult to believe.
"For any of the competitors to have the ability to go tyre testing, using tyres that are going to be used at upcoming grands prix, there is a benefit to that."
Horner said the test would have cost Mercedes hundreds of thousands of dollars in engine mileage, component time, overheads and the logistics of running the car.
"We talk an awful lot on saving costs, yet we spent three hours on Friday talking about in-season testing trying to find a solution," he added. "Now one team has already done a huge amount of it."