My M3 DS had light leaks where the shutter cloth was pulling away from the metal end of the curtain. The hole in the curtain was quite visible when I advanced the film. The curtain had to be replaced. (The camera had not been used for years before I bought it.)
My M4 had light leaks in the curtain that could be seen by shining a flashlight into the front of the camera and looking at the curtain from the rear of the camera, in a dimly lit room. There were dozens of pin-holes in the curtain, even though the curtain looked fine in normal lighting. Using the flashlight, the curtain looked more like a screendoor than a curtain. That curtain had to be replaced, too. (This camera, too, had sat nearly unused for decades before I bought it.)
Get a small but powerful flashlight (torch), turn down the room lights, take the lens off the camera, open the back, and shine the light into the camera's front while looking at the shutter curtains from the back. Use the film wind lever as if advancing the film, and inspect the curtains as they move across the gate. Tiny little holes will show up easily with this method - and they can cause problems. Any light that comes through while winding the advance lever is a leak.
There's information on the web about how to repair small holes with liquid electricians tape, or other liquids that can permanently fill in the hole. Big holes and tears require curtain replacement.