To be on the prowl is an animal reference for walking around the forest or the jungle, moving quietly while hunting, as in search of prey or something to eat.
And, we use the same expression for people who are out, usually at night, looking for a sexual partner or someone to date.
Let’s see some examples :
– Boastfulness is one of the things you must avoid when you’re on the prowl.
– Yesterday was the first time I saw Loyd out after divorce and he was on the prowl.