actually leave his lighter in the office when using it as an excuse to return and wreck the place?

Did he expect Jim to search him??

For me this is the one episode I will never tire of.

I took it with me when I went away on a 4 day Ford training course and, from the comfort of my room, watched it at least three times a day!!!

Oohhh to have met the gorgeous Lily in a
former life ]

I have always wondered the same thing about the lighter. I posed this question a while back. And I believe someone answered that it was really for the viewer's benefit. That we would see him placing the lighter there and it was a good visual.
I can't think of any other possible explanation for it. There was no reason why he actually had to leave it there.

And wowzers!!! 3 times a day!! terrific!!

The thing I have always wondered about this episode is this- If Jim had refused to lie to his wife and had instead told her where he really was (at Ken's cabin), then Ken would have had to abandon the murder. At least for that day, anyway. Jim would then have discovered that Ken had trashed their office and staged a break in. He would surely have been very suspicious of that. He might not have suspected murder but he would have known Ken was up to something. Ken obviously knew Jim well enough to know he would hate lying to his wife and he should have allowed for that possibility in his plan.
I think Murder By The Book is a great episode too, and I am not meaning to pick holes or find fawlts in it; I just enjoy examining the plot in detail.

I agree with the visual theory, it adds to the viewer feeling that 'something is up' that begins when Ken greets Jim at the door with a gun at his face.

I suppose there was the possibility that when Ken told Jim he was going back for his lighter, Jim might have wanted to come along with him rather than wait in the car. If the lighter wouldn't be there, Jim might suspect something. But then in that case, Ken would not have been able to trash the office.