People cross themeselves so often and at so many different times, how the heck did you notice? I don't think it's wrong you noticed, but, dang. Sometimes I just cross myself because . . . I feel like it. Sometimes the priest says something or I may lose focus and try to bring myself back to what's going on, whatever, and I cross myself. I hope you don't come to my church and see me! You would get really confused as to why and when I make the sign of the cross!
I'd like to add that, while I understand it's not proper, it's not like showing up to church in a mini-skirt, jeans or a t-shirt. If it's an honest mistake, so what? Yeah, they should bow. Sometimes I get carried away and make the sign of the cross without even thinking about it. It just happens.
Making the sign of the cross can mean many things, by the way.