Recently, I replaced valve gasket successfully after I found little oil around ignition coil boots.

Later, I noticed dark brown oil spots on the ground, which I guess are leaking engine oil.
But, the engine looks clean. I do not see anything around the valve gasket line. Engine oil level is still good.

It seems that there are oil spots under rather left engine side. I can see them continuously.

What would it come from? I have no lifts, then what should I check first?

Thank you in advance.