Coolant Bubbling in Radiator with Cap off

On my 2001 LS430, I was a having a very slow leak from the radiator drain plug, so I went to a local mechanic to properly seal the plug. While he was checking out the issue. After the car reached the normal temp, he slowly opened the radiator cap (for whatever the reason is). I got shocked that I saw air bubbles (small foamy bubbles) on top of the water inside the radiator.
My car drives completely fine. No any kind of smoke out of exhaust, Not overheating. No CEL, idling smoothly. I even asked the guy to switch off the car and re-start it while the cap is off, it didn’t spit any coolant out. and it didn’t do the bubbling again!
Can this be a head gasket leak?