Its possible, I have often heard vacuum leaks; even inside doors, but not usually. One reason for this is that the source lines act as an oriface/restrictor. The air only makes noise where the velocity is high. I would imagine that the source lines/check valves are often a greater restriction than the leak, so your noise will not be at the leak.

If you have a gauge the method I gave above is simple and PRECISE. If its a door lock problem and the car is over ten to twelve years, I would replace all three door vacuum elements and forget the diagnosis. If you fix only a single unit you will soon be back diagnosing the rest one at a time. If you are paying for diagnosis its cheaper to just replace all of them as they are all the same age.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician