Probably what your problem is a common malady. Rubber products take a "set" over time. When rubber is compressed for a time it looses its elasticity and retains the shape that it was put in. That is why you have to retighten those screw type hose clamps (and why they are not allowed on aircraft hoses). You probably have noticed when you pull a radiator hose that the impression of the clamp does not go away.
Anyway, what happened is that your pan gasket also will take a "set" over time. It is "used" to being compressed in a certain way in a certain place. If you could get it back into the exact spot it was originally and tighten it to th exact same tightness then it would not leak, but the reality is that is almost never possible. Therefore, a small seep leak will develop.
Solution, use a new gasket. (I would still use a very thin smear of gasket sealer on one side to hold the gasket in place.) Or, use a very thin smearing of sealer or liquid gasket material with the old gasket. In your case, with the old gasket securely held on the block I would have just made sure it and the pan were perfectly clean and free of oil, smeared a sealer on the pan and put it together. I cannot guaranty that you would not have had a leak, but I am pretty confident that would be true.
One word about gasket sealer and silicone. More is NOT better. if you think that there might be too much there most definitely is. Remember that you are filling a VERY thin space and any excess will either go to the inside or outside. On the inside it can collect on screens, and block small openings. (I have found bits of silicone in radiators and for sure transmission/hydraulic pick up tube screens.) On the outside you have an unsightly red, blue, or black tell-tale band around the part that you had off.
Mike