Was L suspicious of all Light's girlfriends or only Misa?
I feel like it could've just been the way Misa acted toward's Light... That and the fact that she was a very popular model talking to a regular student. He could've seen how bubbly her personality is and how much she doesnt fit into Light's "no nonsense" studios type of personality. If you can tell he doesnt really want to be around her then it wouldnt be a far reach to assume that she's around for some other reason.