LONDON, England - The real mystery about Pirelli's “secret” tyre test with Mercedes that’s been stirring up track dust for the past 24 hours is why it took the rest of Formula 1 so long to find out about it.The “shock revelation” was the talk of the 2013 Monaco GP weekend, with Ferrari and 2013 champion 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 F1 pits 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.TESTING BANNED FOR YEARSTeams 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 motor homes.In-season testing has been banned for years, a move agreed by all teams to keep costs down, although Pirelli can conduct its own limited tyre tests with an older car.Yet Mercedes and Pirelli managed to hold a three-day, 1000km test at Barcelona's Catalunya circuit in the week directly after the 2013 Spanish F1 GP with a 2013 car without anybody noticing. Mercedes suggested that was hardly its fault.The brand’s motorsport head Toto Wolff said: "We left everything there. Garages, buses, trucks, engineering offices. Nothing was secret. We didn't Twitter... should we have?"EVERYBODY WEARING A BLINDFOLD?Dismantling a multi-storey motor home such as the glass-fronted one used by McLaren or the “energy station” (it’s an “energy” drink, silly) favoured by World champion Red Bull can take days.Surely, paddock insiders wondered, there must have been somebody 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 that 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 is good at holding a secret. Most other things you hear about, somebody is going there or leaving, but they managed to keep this one remarkably quiet.”Mercedes could now face heavy sanctions. If the Monaco-winning team is found to have flouted the regulations it could be stripped of points, fined - even excluded.'WE'LL DO THE SAME'Should it 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 has pushed repeatedly for in-season testing to be re-introduced and team principal Stefano Domenicali said that was one of the reasons for its protest. He told reporters: "If this is possible we will be the first to raise our arms to make sure we can do the same because Ferrari has always been very much pushing to try to do in-season track testing on the track.”Apart from resolving the legality or otherwise of what Mercedes did - and Mercedes is adamant that it has done nothing wrong - considerable damage has already been done.Horner and other principals expressed concern over the lack of transparency; they say neither Pirelli nor Mercedes notified other teams about the test. They 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 June when modified Pirelli tyres are due to be introduced.'WHEN YOU RUN, YOU'RE LEARNING'Voices in the paddock were already suggesting on Sunday (May 26) that any Mercedes championship challenge could be considered tainted, even if Rosberg's Monaco victory was flagged up long before the test, given his pole positions in Bahrain and Spain.Horner said Mercedes' 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 it 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 about saving costs, yet we spend 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." Stay with Wheels24 for the 2013 F1 season – fresh reports every day.