OK we all know that it happened. Here is how I think he did it.
1. Side of the museum with bullet holes.
2. Area not highly traveled at night or early morning.
3. Area from where the shots were taken from.
It would be easy to park in the area of #2 and slip into the woods and walk along I-95 without being seen. The area marked #3 is where the freeway was cut through a small hill. Offering the best vantage point on the museum.