Thanks for that Scrabbler1, it answers my query perfectly. Jerry Orbach is missed. He was one of the great characters of US TV.

I don't know for sure if this happened the way I hoped it would, but I have a small story which may answer your question.
Back in the late 1990s, when A&E held the rights to L&O, we had a message board like this one for L&O fans. Once, an A&E moderator posted a thread called, "Ask Jerry" where we could post questions for Jerry Orbach by a certain day and he would answer them. I asked the question you raised and his answer was this: During the script read-through, he would ad-lib a funny quip once in a while. If the writers liked it, and they sometimes did, then they would leave it in the script. He didn't cite any examples which would have been a nice touch.
Also back in those years, I owned a VCR and taped the episodes from the first ~10 seasons, later creating several Word files with Lennie's funny lines. I managed to retain those files.