Yes but this is not exactly a secret back door to the Vatican. It is an odd tradition at St. Peter's involving the Porta Santa, or Holy Door. This door is in the front of the basilica to the right of the main entrance. Most of the time it's kept not merely locked but walled up. It's opened only during Holy Years, also known as Jubilee Years. Massive numbers of pilgrims descend on St. Peter's at these times, and I gather the door functions as a sort of Holy Fire Exit.
Holy Years are an odd tradition in their own right. The first was proclaimed in AD 1300 by Pope Boniface VIII, not entirely voluntarily. The faithful somehow got the idea that centenary years were the occasion of a Great Pardon. Tens of thousands of them spontaneously embarked on a pilgrimage to Rome with the view of getting one.
So here's Boniface looking out the window, and he sees vast crowds of people who'd evidently done something bad enough that they figured it was worth going to Rome to get a pardon for. Whoa, says Boniface, time to think fast.
He worked up a system whereby participants could gain a special indulgence (pardon from punishment for sins) in return for fulfilling various conditions, notably visiting certain Roman churches, St. Peter's being the most important.
The original plan was that Holy Years would occur every 100 years, but the interval was soon reduced to 25 years. Additional Holy Years are sometimes proclaimed for special occasions, e.g., in 1983-'84, which marked the 1,950th anniversary of the Crucifixion and Resurrection.
A Holy Year starts on Christmas Eve, at one time considered the last day of the year. There's an elaborate ritual in which the pope strikes the Holy Door three times with a silver hammer. The door promptly collapses, no doubt inspiring at least one or two spectators to hope that the rest of St. Peter's was built by a different contractor. In fact, however, workers with ropes and pulleys nudge things along.
Considering all the buildup, one would suppose the Holy Door provided admittance to a garden of forbidden delights. But in fact it gets you into the back of the church just as the other entrances do. The door remains open until the following Christmas Eve, when it's again walled up.
One can appreciate that having a special door heightens the drama of the Holy Year, provides instructive symbolism, etc. But why the pope feels he has to wall it up as opposed to using a good dead bolt is a matter that remains obscure. Cynics will of course suggest that some cardinal's nephew probably has the plaster contract.