Ideas on my leak?

[Lobo'74]

Truck has been sitting for about a month or so. I started it up , let it idle for a couple of minutes moving another car off the driveway, then drove about a mile to the gas station. Found fresh engine coolant in the "dimple" in the left front of my intake manifold, and a small line of fluid all down the left engine/intake manifold/valve cover area. No visible leak. I dried it all off and ran it for another 5 - 7 miles or so and checked it again... Still no more leak.

The engine is a '79 350 so it has the thermostat cover on top of the engine in this same area. Any chance that after sitting in the garage for a month that the gasket around the thermostat dried out and allowed the leak until it swelled up again?

No fluid in the oil so i don't think its a cracked block, but is is weird.

Any ideas?