Furnace smell and stains

Wow! I only get furnaces that heat.
Just having fun.

Manufacturer, Model Number - MN, and Seral Number - SN, please.

Kevin. Typically odors can be from fuel leaks or lack of proper combustion.
Purchase a combustible gas leak detector.

The stains below the manifold might be condensate.
Temp and humidity in the furnace room?

The furnace likely operated when you approach the cabinet because movement displaced the air around the sensor and triggered the furnace to heat. No worries. It happened to me every now and then.

Awaiting your reply.

I am with you on this.

As for NG leaks.
If there is a leak it’s best to inform all involved. I have never been treated badly.