I have been observing him since I wasn't on resign.

[emsr2d2]

No, that makes no sense. Do you mean "I was already watching my team leader's behaviour before I handed in my notice"?

 

[Tarheel]

Perhaps:

I was observing my team leader's behavior since I started started serving the notice period.

Or:

I had been observing my team leader's behavior since I started serving the notice period.

 

[tufguy]

No, that makes no sense. Do you mean "I was already watching my team leader's behaviour before I handed in my notice"?

Yes.